/* ===================================================================
   Hull Notes — styles (apple.com-style visual layer; content untouched)
   =================================================================== */
:root {
  color-scheme: light;
  --bg: #f5f5f7;
  --surface: #ffffff;
  --surface-2: #f5f5f7;
  --surface-3: #e8e8ed;
  --text: #1d1d1f;
  --text-2: #424245;
  --muted: #6e6e73;
  --border: #e5e5ea;
  --border-2: #d2d2d7;
  --accent: #0071e3;
  --accent-ink: #0066cc;
  --accent-soft: #e8f2fd;
  --btn: #0071e3; --btn-hover: #0077ed; --eyebrow: #bf4800;
  --fill: rgba(118,118,128,.12); --seg-on: #ffffff;
  --nav-bg: rgba(250,250,252,.8); --nav-ink: rgba(0,0,0,.8); --nav-line: rgba(0,0,0,.08); --side-bg: rgba(255,255,255,.72);
  --good: #248a3d; --good-soft: #e9f6ec;
  --bad: #d70015; --bad-soft: #fdecee;
  --warn: #b25000; --warn-soft: #fff4e5;
  --tip-soft: #eef5fe; --q-soft: #f6f0fd; --q: #8944ab; --ex-soft: #ebf7ef; --ex: #248a3d;
  --hl: #fff3a8;
  --in: #fffbe6; --out: #e9f6ec;
  --s1: #0071e3; --s2: #ff9500; --s3: #28cd41; --s4: #ffcc00; --s5: #ff2d55; --s6: #30b0c7; --s7: #af52de; --s8: #ff3b30;
  --shadow: 0 1px 2px rgba(0,0,0,.03), 0 4px 18px rgba(0,0,0,.05);
  --shadow-hover: 0 2px 6px rgba(0,0,0,.05), 0 14px 36px rgba(0,0,0,.10);
  --radius: 18px;
  --font: -apple-system, BlinkMacSystemFont, 'SF Pro Text', 'SF Pro Display', 'Inter', 'Helvetica Neue', Helvetica, Arial, 'PingFang SC', 'Noto Sans SC', sans-serif;
  --serif: -apple-system, BlinkMacSystemFont, 'SF Pro Display', 'Inter', 'Helvetica Neue', Helvetica, Arial, 'PingFang SC', 'Noto Sans SC', sans-serif;
  --mono: 'SF Mono', ui-monospace, SFMono-Regular, Menlo, 'JetBrains Mono', monospace;
  --side: 288px; --right: 230px; --top: 52px;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    color-scheme: dark;
    --bg: #000000; --surface: #1d1d1f; --surface-2: #2a2a2d; --surface-3: #3a3a3c;
    --text: #f5f5f7; --text-2: #d1d1d6; --muted: #8e8e93; --border: #333336; --border-2: #48484a;
    --accent: #0a84ff; --accent-ink: #2997ff; --accent-soft: #0c2a4a;
    --btn: #0071e3; --btn-hover: #0077ed; --eyebrow: #ff791b;
    --fill: rgba(118,118,128,.24); --seg-on: #636366;
    --nav-bg: rgba(22,22,23,.8); --nav-ink: rgba(255,255,255,.82); --nav-line: rgba(255,255,255,.08); --side-bg: rgba(22,22,23,.72);
    --good: #30d158; --good-soft: #0f2e19; --bad: #ff453a; --bad-soft: #3b1513; --warn: #ff9f0a; --warn-soft: #33230a;
    --tip-soft: #0d1f33; --q-soft: #241a33; --q: #bf5af2; --ex-soft: #0e2a19; --ex: #30d158;
    --hl: #5c4a00; --in: #2b2812; --out: #12301c;
    --s1: #0a84ff; --s2: #ff9f0a; --s3: #30d158; --s4: #ffd60a; --s5: #ff375f; --s6: #64d2ff; --s7: #bf5af2; --s8: #ff453a;
    --shadow: 0 0 0 1px rgba(255,255,255,.04);
    --shadow-hover: 0 0 0 1px rgba(255,255,255,.08), 0 10px 30px rgba(0,0,0,.5);
  }
}
:root[data-theme="dark"] {
  color-scheme: dark;
  --bg: #000000; --surface: #1d1d1f; --surface-2: #2a2a2d; --surface-3: #3a3a3c;
  --text: #f5f5f7; --text-2: #d1d1d6; --muted: #8e8e93; --border: #333336; --border-2: #48484a;
  --accent: #0a84ff; --accent-ink: #2997ff; --accent-soft: #0c2a4a;
  --btn: #0071e3; --btn-hover: #0077ed; --eyebrow: #ff791b;
  --fill: rgba(118,118,128,.24); --seg-on: #636366;
  --nav-bg: rgba(22,22,23,.8); --nav-ink: rgba(255,255,255,.82); --nav-line: rgba(255,255,255,.08); --side-bg: rgba(22,22,23,.72);
  --good: #30d158; --good-soft: #0f2e19; --bad: #ff453a; --bad-soft: #3b1513; --warn: #ff9f0a; --warn-soft: #33230a;
  --tip-soft: #0d1f33; --q-soft: #241a33; --q: #bf5af2; --ex-soft: #0e2a19; --ex: #30d158;
  --hl: #5c4a00; --in: #2b2812; --out: #12301c;
  --s1: #0a84ff; --s2: #ff9f0a; --s3: #30d158; --s4: #ffd60a; --s5: #ff375f; --s6: #64d2ff; --s7: #bf5af2; --s8: #ff453a;
  --shadow: 0 0 0 1px rgba(255,255,255,.04);
  --shadow-hover: 0 0 0 1px rgba(255,255,255,.08), 0 10px 30px rgba(0,0,0,.5);
}
* { box-sizing: border-box; }
html { scroll-padding-top: calc(var(--top) + 16px); }
body { margin: 0; background: var(--bg); color: var(--text); font: 15.5px/1.7 var(--font); -webkit-font-smoothing: antialiased; }
a { color: var(--accent-ink); text-decoration: none; }
a:hover { text-decoration: underline; }
code, kbd { font-family: var(--mono); font-size: .86em; background: var(--surface-2); border: 1px solid var(--border); border-radius: 5px; padding: 0 .35em; }
kbd { border-bottom-width: 2px; }
h1, h2, h3, h4 { line-height: 1.3; }
h1 { font-family: var(--serif); font-size: 2.1rem; margin: .2em 0 .3em; letter-spacing: -.01em; }
.muted { color: var(--muted); } .small { font-size: .85em; }
.lead { font-size: 1.08rem; color: var(--text-2); max-width: 68ch; }
mark { background: var(--hl); color: inherit; border-radius: 3px; padding: 0 2px; }
#scrollbar { position: fixed; top: 0; left: 0; height: 3px; background: var(--accent); z-index: 60; width: 0; transition: width .1s; }

/* ---------------- language modes: English (default) vs English + Chinese keywords */
:root:not([data-lang="bi"]) .zh, :root:not([data-lang="bi"]) .zh-only { display: none !important; }
:root[data-lang="bi"] .zh { color: var(--accent-ink); background: var(--accent-soft); border-color: color-mix(in srgb, var(--accent) 25%, transparent); }
:root[data-lang="bi"] .zh { margin-left: .35em; }
.lang-sel { font: 600 13px var(--font); color: var(--text); background: var(--surface-2); border: 1px solid var(--border-2); border-radius: 8px; padding: 6px 8px; cursor: pointer; }
.lang-sel:focus { outline: 2px solid var(--accent); outline-offset: -1px; }
.en-sub { display: block; font-size: 11.5px; color: var(--muted); font-weight: 400; font-style: italic; }
:root[data-lang="zh"] .sec-zh { font-style: italic; font-size: .85rem; }
.lang-btn { width: auto !important; padding: 0 10px; font: 600 13px var(--font) !important; border: 1px solid var(--border-2) !important; white-space: nowrap; }
.lang-btn.on { background: var(--accent-soft); color: var(--accent-ink); border-color: var(--accent) !important; }
.lang-zh { font-weight: 500; }

/* ---------------- top bar */
.top { position: fixed; inset: 0 0 auto 0; height: var(--top); display: flex; align-items: center; gap: 10px; padding: 0 14px; background: color-mix(in srgb, var(--surface) 88%, transparent); backdrop-filter: blur(10px); border-bottom: 1px solid var(--border); z-index: 50; }
.brand { display: flex; align-items: center; gap: 10px; color: var(--text); font-weight: 700; white-space: nowrap; }
.brand:hover { text-decoration: none; }
.brand small { display: block; font-weight: 400; font-size: 11px; color: var(--muted); line-height: 1.1; }
.brand > span:last-child { line-height: 1.2; }
.brand-mark { width: 32px; height: 32px; border-radius: 8px; background: var(--accent); color: #fff; display: grid; place-items: center; font: 700 20px var(--serif); }
.topnav { display: flex; gap: 2px; margin-left: 12px; overflow-x: auto; scrollbar-width: none; }
.topnav a { color: var(--text-2); padding: 6px 10px; border-radius: 8px; font-size: 14px; white-space: nowrap; }
.topnav a:hover, .topnav a.on { background: var(--surface-2); color: var(--text); text-decoration: none; }
.search { margin-left: auto; }
.search input { width: 220px; padding: 7px 12px; border-radius: 9px; border: 1px solid var(--border); background: var(--surface-2); color: var(--text); font: inherit; font-size: 14px; }
.search input:focus { outline: 2px solid var(--accent); outline-offset: -1px; background: var(--surface); }
.icon-btn { background: none; border: 1px solid transparent; color: var(--text-2); font-size: 18px; width: 36px; height: 36px; border-radius: 8px; cursor: pointer; }
.icon-btn:hover { background: var(--surface-2); }
#menu-btn { display: none; }

/* ---------------- sidebar */
.side { position: fixed; top: var(--top); bottom: 0; left: 0; width: var(--side); overflow-y: auto; border-right: 1px solid var(--border); background: var(--surface); padding: 10px 8px 40px; z-index: 40; scrollbar-width: thin; }
.toc-part { margin-bottom: 8px; }
.toc-part-name { font-size: 11px; text-transform: uppercase; letter-spacing: .06em; color: var(--muted); font-weight: 600; padding: 10px 10px 4px; }
.toc-part-name small { display: block; text-transform: none; letter-spacing: 0; font-weight: 400; }
.toc-ch { display: flex; align-items: center; gap: 8px; padding: 6px 10px; border-radius: 8px; color: var(--text-2); font-size: 13.5px; line-height: 1.3; }
.toc-ch:hover { background: var(--surface-2); text-decoration: none; color: var(--text); }
.toc-ch.active { background: var(--accent-soft); color: var(--accent-ink); font-weight: 600; }
.toc-num { width: 22px; text-align: right; color: var(--muted); font-variant-numeric: tabular-nums; flex: none; font-size: 12px; }
.toc-t { flex: 1; }
.toc-t small { display: block; color: var(--muted); font-size: 11.5px; font-weight: 400; }
.ring { flex: none; }
.ring-bg { fill: none; stroke: var(--border); stroke-width: 3; }
.ring-fg { fill: none; stroke: var(--accent); stroke-width: 3; stroke-linecap: round; }
.ring.done .ring-fg { stroke: var(--good); }
#scrim { display: none; }

/* ---------------- main */
.main { margin-left: var(--side); padding: calc(var(--top) + 28px) 40px 120px; max-width: calc(var(--side) + 980px); }
body.has-right .main { margin-right: 0; }
@media (min-width: 1480px) { body.has-right .main { max-width: calc(var(--side) + 940px); } }
.main.chapter, .main.toolview { max-width: calc(var(--side) + 960px); }
.loading { color: var(--muted); padding: 40px 0; }
.rightnav { display: none; }
@media (min-width: 1480px) {
  body.has-right .rightnav { display: block; position: fixed; top: calc(var(--top) + 24px); right: 18px; width: var(--right); max-height: calc(100vh - var(--top) - 48px); overflow-y: auto; font-size: 12.5px; scrollbar-width: thin; }
  body.has-right .main { margin-right: calc(var(--right) + 30px); }
}
.rn-title { font-weight: 600; text-transform: uppercase; font-size: 11px; letter-spacing: .06em; color: var(--muted); margin-bottom: 6px; }
.rightnav a { display: flex; gap: 6px; color: var(--text-2); padding: 3px 8px; border-left: 2px solid var(--border); line-height: 1.35; }
.rightnav a span { color: var(--muted); min-width: 30px; font-variant-numeric: tabular-nums; }
.rightnav a:hover { color: var(--text); text-decoration: none; border-color: var(--border-2); }
.rightnav a.cur { border-color: var(--accent); color: var(--accent-ink); font-weight: 600; }
.rightnav a.done span::after { content: ' ✓'; color: var(--good); }
body.focus .side, body.focus .rightnav { display: none !important; }
body.focus .main { margin-left: auto; margin-right: auto !important; max-width: 980px; }

/* ---------------- buttons, chips */
.btn { display: inline-flex; align-items: center; gap: 6px; padding: 8px 15px; border-radius: 9px; border: 1px solid var(--accent); background: var(--accent); color: #fff; font: 600 14px var(--font); cursor: pointer; text-decoration: none !important; white-space: nowrap; }
.btn:hover { filter: brightness(1.06); }
.btn.ghost { background: var(--surface); color: var(--text); border-color: var(--border-2); }
.btn.ghost:hover { background: var(--surface-2); }
.btn.sm { padding: 4px 10px; font-size: 13px; border-radius: 7px; }
.chip { display: inline-block; padding: 3px 10px; border-radius: 99px; border: 1px solid var(--border-2); color: var(--text-2); font-size: 13px; margin: 2px; }
.chip.on { background: var(--accent); border-color: var(--accent); color: #fff; }
.seg { display: inline-flex; border: 1px solid var(--border-2); border-radius: 9px; overflow: hidden; }
.seg button { border: 0; background: var(--surface); color: var(--text-2); padding: 6px 12px; font: 500 13px var(--font); cursor: pointer; border-right: 1px solid var(--border); }
.seg button:last-child { border-right: 0; }
.seg button.on { background: var(--accent-soft); color: var(--accent-ink); font-weight: 600; }
.card { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 18px 20px; box-shadow: var(--shadow); }
.card h3 { margin: 0 0 8px; font-size: 1rem; }

/* ---------------- home */
.hero { display: grid; grid-template-columns: 1.6fr 1fr; gap: 28px; align-items: end; padding: 10px 0 24px; border-bottom: 1px solid var(--border); margin-bottom: 22px; }
.hero h1 { font-size: 2.5rem; margin: .15em 0 .25em; }
.kicker { text-transform: uppercase; letter-spacing: .08em; font-size: 12px; color: var(--accent-ink); font-weight: 600; }
.hero-actions { display: flex; gap: 8px; flex-wrap: wrap; margin-top: 16px; }
.stats { display: grid; grid-template-columns: 1fr 1fr; gap: 10px; }
.stats > div { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 12px 14px; }
.stats b { display: block; font-size: 1.7rem; font-variant-numeric: tabular-nums; line-height: 1.2; }
.stats span { font-size: 12.5px; color: var(--muted); }
.home-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; margin-bottom: 10px; }
.plan-form { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; }
.plan-form input { font: inherit; padding: 5px 8px; border: 1px solid var(--border-2); border-radius: 7px; background: var(--surface); color: var(--text); }
.plan-row { display: flex; justify-content: space-between; margin-bottom: 6px; }
.plan-row .good { color: var(--good); font-weight: 600; } .plan-row .bad { color: var(--bad); font-weight: 600; }
.plan-bar { position: relative; height: 12px; background: var(--surface-2); border-radius: 6px; overflow: hidden; margin-bottom: 8px; }
.plan-bar i { position: absolute; left: 0; top: 0; bottom: 0; border-radius: 6px; }
.plan-bar .exp { background: var(--border-2); } .plan-bar .act { background: var(--accent); height: 6px; top: 3px; }
.heat { display: grid; grid-auto-flow: column; grid-template-rows: repeat(7, 11px); gap: 3px; margin-bottom: 6px; overflow-x: auto; }
.heat i { width: 11px; height: 11px; border-radius: 2px; background: var(--surface-2); }
.heat i.l1 { background: #cde2fb; } .heat i.l2 { background: #86b6ef; } .heat i.l3 { background: #3987e5; } .heat i.l4 { background: #1c5cab; }
.part-h { font-family: var(--serif); font-size: 1.3rem; margin: 30px 0 12px; }
.part-h small { font-family: var(--font); font-size: .8rem; color: var(--muted); font-weight: 400; margin-left: 6px; }
.ch-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(200px, 1fr)); gap: 12px; }
.ch-card { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 14px; color: var(--text); transition: transform .12s, box-shadow .12s, border-color .12s; }
.ch-card:hover { text-decoration: none; transform: translateY(-2px); box-shadow: var(--shadow); border-color: var(--border-2); }
.cc-top { display: flex; justify-content: space-between; align-items: center; }
.cc-num { font: 700 1.4rem var(--serif); color: var(--accent-ink); }
.cc-title { font-weight: 600; margin-top: 6px; line-height: 1.3; }
.cc-zh { color: var(--text-2); font-size: 13px; }
.cc-meta { color: var(--muted); font-size: 12px; margin-top: 6px; }
.tool-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); gap: 12px; }
.tool-card { display: flex; flex-direction: column; gap: 3px; background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 14px; color: var(--text); position: relative; }
.tool-card:hover { text-decoration: none; border-color: var(--accent); }
.tool-card small { color: var(--muted); line-height: 1.4; }
.tool-card em { position: absolute; top: 12px; right: 12px; font-style: normal; font-size: 11px; color: var(--muted); }
.tc-icon { font-size: 22px; }

/* ---------------- chapter */
.ch-head { padding-bottom: 16px; border-bottom: 1px solid var(--border); margin-bottom: 18px; }
.ch-kicker { color: var(--accent-ink); font-weight: 600; font-size: 13px; text-transform: uppercase; letter-spacing: .06em; }
.ch-kicker .dot { color: var(--muted); margin: 0 4px; }
.ch-head h1 { font-size: 2.3rem; margin: .15em 0 0; }
.ch-zh { font-size: 1.15rem; color: var(--text-2); margin-bottom: 12px; }
.ch-progress { display: flex; align-items: center; gap: 12px; flex-wrap: wrap; font-size: 13px; color: var(--muted); }
.ch-progress .bar, .fc-stats .bar { width: 180px; height: 6px; background: var(--surface-2); border-radius: 3px; overflow: hidden; }
.ch-progress .bar i, .fc-stats .bar i { display: block; height: 100%; background: var(--good); border-radius: 3px; transition: width .3s; }
.intro { font-size: 1.04rem; color: var(--text-2); }
.terms-box { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 10px 16px; margin: 16px 0; }
.terms-box summary { cursor: pointer; font-weight: 600; }
.terms { display: grid; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr)); gap: 10px 20px; margin: 12px 0 4px; }
.terms dt { font-weight: 600; } .terms dd { margin: 0; color: var(--text-2); font-size: 14px; }
.sec { background: var(--surface); border: 1px solid var(--border); border-radius: 14px; padding: 6px 28px 18px; margin: 22px 0; box-shadow: var(--shadow); position: relative; }
.sec.is-read { border-left: 4px solid var(--good); }
.sec h2 { font-family: var(--serif); font-size: 1.45rem; margin: 18px 0 10px; display: flex; align-items: baseline; gap: 10px; flex-wrap: wrap; }
.sec-id { font-family: var(--font); font-size: .78rem; font-weight: 700; color: var(--accent-ink); background: var(--accent-soft); padding: 2px 8px; border-radius: 6px; font-variant-numeric: tabular-nums; }
.sec-zh { font-family: var(--font); font-size: .95rem; color: var(--muted); font-weight: 500; }
.sec-link { margin-left: auto; color: var(--border-2); font-size: 1rem; }
.sec:hover .sec-link { color: var(--muted); }
.sec-body { overflow-wrap: break-word; }
.sec-body > *:first-child { margin-top: 0; }
.sec-body h4 { margin: 1.3em 0 .4em; font-size: 1.03rem; }
.sec-body ul, .sec-body ol { padding-left: 1.4em; }
.sec-body li { margin: .2em 0; }
.sec-body img, .sec-body svg { max-width: 100%; }
.zh { display: inline-block; font-size: .86em; background: var(--surface-2); color: var(--text-2); border-radius: 5px; padding: 0 6px; margin: 0 2px; font-weight: 500; border: 1px solid var(--border); line-height: 1.5; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: 4px 0; }
.formula { background: var(--surface-2); border-radius: 10px; padding: 6px 16px; margin: 12px 0; border: 1px solid var(--border); }
.formula .where { font-size: 14px; color: var(--text-2); padding-bottom: 6px; }
.callout { border-radius: 10px; padding: 10px 14px; margin: 14px 0; border: 1px solid var(--border); background: var(--surface-2); }
.callout > *:first-child { margin-top: 0; } .callout > *:last-child { margin-bottom: 0; }
.callout.tip { background: var(--tip-soft); border-color: color-mix(in srgb, var(--accent) 25%, transparent); }
.callout.warn { background: var(--warn-soft); border-color: color-mix(in srgb, var(--warn) 30%, transparent); }
.callout.def { background: var(--surface-2); border-left: 4px solid var(--text-2); }
.callout.example { background: var(--ex-soft); border-color: color-mix(in srgb, var(--ex) 30%, transparent); }
.callout.key { background: var(--tip-soft); border-left: 4px solid var(--accent); }
.callout.exam { background: var(--warn-soft); border-left: 4px solid var(--warn); }
.callout.exam > b:first-child { color: var(--warn); text-transform: uppercase; letter-spacing: .04em; font-size: .85em; }
.callout.exam ul { margin: 4px 0 0; }
/* English technical-term chip in the Chinese mode */
.en { display: inline-block; font-size: .8em; font-style: italic; color: var(--text-2); background: var(--surface-2); border: 1px solid var(--border); border-radius: 5px; padding: 0 5px; margin-left: .3em; line-height: 1.5; font-weight: 500; }
:root[data-lang="zh"] body { font-family: 'Noto Sans SC', 'Inter', -apple-system, 'PingFang SC', sans-serif; }
.callout.q { background: var(--q-soft); border-color: color-mix(in srgb, var(--q) 30%, transparent); }
.callout.q > b:first-child { color: var(--q); }
.callout details { margin-top: 6px; }
.callout summary { cursor: pointer; color: var(--q); font-weight: 600; font-size: 14px; }
.tbl { border-collapse: collapse; width: 100%; margin: 12px 0; font-size: 14px; display: block; overflow-x: auto; }
.tbl th, .tbl td { border: 1px solid var(--border); padding: 6px 10px; text-align: left; vertical-align: top; }
.tbl thead th { background: var(--surface-2); font-weight: 600; }
.tbl td.n, .tbl th.n { text-align: right; font-variant-numeric: tabular-nums; }
.sec-foot { display: flex; align-items: center; gap: 10px; margin-top: 14px; padding-top: 12px; border-top: 1px dashed var(--border); }
.read-toggle { display: inline-flex; align-items: center; gap: 6px; font-size: 14px; cursor: pointer; color: var(--text-2); user-select: none; }
.read-toggle input { width: 17px; height: 17px; accent-color: var(--good); }
.sec.is-read .read-toggle { color: var(--good); font-weight: 600; }
.mynote textarea { width: 100%; min-height: 110px; margin-top: 10px; font: 14px/1.6 var(--font); padding: 10px 12px; border: 1px solid var(--border-2); border-radius: 9px; background: var(--in); color: var(--text); resize: vertical; }
.fgrid { display: grid; grid-template-columns: repeat(auto-fill, minmax(300px, 1fr)); gap: 12px; margin-bottom: 10px; }
.fcard { background: var(--surface); border: 1px solid var(--border); border-radius: 10px; padding: 10px 14px; position: relative; }
.formulas-sec .fcard { background: var(--surface-2); }
.fname { font-weight: 600; font-size: 13.5px; }
.ftex { overflow-x: auto; }
.fnote { font-size: 12.5px; color: var(--text-2); }
.copy-tex { position: absolute; top: 8px; right: 8px; border: 0; background: none; color: var(--muted); cursor: pointer; }
.prevnext { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin-top: 30px; }
.pn { display: block; padding: 14px 18px; border: 1px solid var(--border); border-radius: var(--radius); background: var(--surface); color: var(--text); font-weight: 600; }
.pn small { display: block; color: var(--muted); font-weight: 400; }
.pn.next { text-align: right; }
.pn:hover { border-color: var(--accent); text-decoration: none; }
.crumbs { font-size: 13px; color: var(--muted); margin-bottom: 10px; }

/* ---------------- quizzes */
.quiz { margin-top: 18px; border: 1px solid var(--border); border-radius: 12px; padding: 4px 16px 10px; background: var(--surface-2); }
.quiz-head { display: flex; justify-content: space-between; font-weight: 700; padding: 10px 0 4px; font-size: 14px; }
.quiz-score { color: var(--good); font-weight: 600; }
.q { padding: 12px 0; border-top: 1px solid var(--border); }
.quiz-head + .q { border-top: 0; }
.q-text { display: flex; gap: 10px; font-weight: 500; }
.q-text > div { flex: 1; min-width: 0; }
.q-n { flex: none; width: 24px; height: 24px; border-radius: 50%; background: var(--surface-3); display: grid; place-items: center; font-size: 12px; font-weight: 700; margin-top: 2px; }
.q.right .q-n { background: var(--good); color: #fff; } .q.wrong .q-n { background: var(--bad); color: #fff; }
.q-opts { display: grid; gap: 6px; margin: 10px 0 0 34px; }
.q-opts.tf { grid-template-columns: auto auto; justify-content: start; }
.q-opt { display: flex; align-items: flex-start; gap: 10px; padding: 7px 12px; border: 1px solid var(--border); border-radius: 9px; background: var(--surface); cursor: pointer; font-size: 14.5px; }
.q-opt:hover { border-color: var(--border-2); }
.q-opt input { margin-top: 5px; accent-color: var(--accent); }
.q-opt:has(input:checked) { border-color: var(--accent); background: var(--accent-soft); }
.q-opt.is-answer { border-color: var(--good); background: var(--good-soft); }
.q-letter { font-weight: 700; color: var(--muted); }
.q-num { margin: 10px 0 0 34px; display: flex; align-items: center; gap: 8px; }
.q-num input { width: 180px; padding: 7px 10px; border: 1px solid var(--border-2); border-radius: 8px; font: 15px var(--mono); background: var(--surface); color: var(--text); }
.q-num input:focus { outline: 2px solid var(--accent); outline-offset: -1px; }
.q-unit { color: var(--muted); }
.q-actions { margin: 10px 0 0 34px; display: flex; gap: 8px; }
.q-actions.shake { animation: shake .35s; }
@keyframes shake { 25% { transform: translateX(-4px); } 75% { transform: translateX(4px); } }
.q-feedback { margin: 10px 0 0 34px; padding: 10px 14px; border-radius: 9px; background: var(--surface); border: 1px solid var(--border); font-size: 14.5px; }
.q.right .q-feedback { border-color: color-mix(in srgb, var(--good) 40%, transparent); }
.q.wrong .q-feedback { border-color: color-mix(in srgb, var(--bad) 40%, transparent); }
.fb-head { font-weight: 700; margin-bottom: 4px; } .fb-head.ok { color: var(--good); } .fb-head.no { color: var(--bad); }
.fb-ans { margin-bottom: 4px; }
.fb-exp { color: var(--text-2); }
.exam-sec { border: 2px solid var(--accent); }
.exam-head { display: flex; justify-content: space-between; align-items: center; gap: 16px; padding: 14px 0 8px; }
.exam-head h3 { margin: 0; font-family: var(--serif); font-size: 1.4rem; }
.exam .q { border-top: 1px solid var(--border); }
.exam-actions { display: flex; gap: 8px; flex-wrap: wrap; padding-top: 14px; border-top: 1px solid var(--border); }
.score-big { font: 700 2rem var(--serif); text-align: right; }
.score-big.good { color: var(--good); } .score-big.mid { color: var(--warn); } .score-big.low { color: var(--bad); }

/* ---------------- spreadsheet */
.hs-wrap { border: 1px solid var(--border-2); border-radius: 10px; background: var(--surface); margin: 16px 0; overflow: hidden; box-shadow: var(--shadow); }
.hs-head { display: flex; justify-content: space-between; align-items: center; background: #1d6f42; color: #fff; padding: 5px 8px 5px 12px; gap: 8px; flex-wrap: wrap; }
.hs-title { font-weight: 600; font-size: 13.5px; display: flex; gap: 8px; align-items: center; }
.hs-logo { background: #fff; color: #1d6f42; border-radius: 4px; width: 20px; height: 20px; display: grid; place-items: center; font-size: 14px; font-weight: 700; }
.hs-tools { display: flex; gap: 3px; flex-wrap: wrap; }
.hs-tools button { background: rgba(255,255,255,.12); color: #fff; border: 1px solid rgba(255,255,255,.25); border-radius: 5px; font: 500 12px var(--font); padding: 2px 8px; cursor: pointer; }
.hs-tools button:hover, .hs-tools button.on { background: rgba(255,255,255,.3); }
.hs-desc { padding: 8px 12px; font-size: 13.5px; color: var(--text-2); border-bottom: 1px solid var(--border); background: var(--surface-2); }
.hs-bar { display: flex; align-items: center; border-bottom: 1px solid var(--border); background: var(--surface); }
.hs-name { width: 86px; border: 0; border-right: 1px solid var(--border); padding: 5px 8px; font: 12.5px var(--mono); background: transparent; color: var(--text); }
.hs-fx { color: var(--muted); font-style: italic; padding: 0 8px; border-right: 1px solid var(--border); font-family: var(--serif); }
.hs-formula { flex: 1; border: 0; padding: 5px 8px; font: 13px var(--mono); background: transparent; color: var(--text); min-width: 0; }
.hs-name:focus, .hs-formula:focus { outline: 2px solid #1d6f42; outline-offset: -2px; }
.hs-scroll { overflow: auto; max-height: 460px; outline: none; position: relative; }
.hs-grid { border-collapse: separate; border-spacing: 0; table-layout: fixed; font: 13px/1.2 var(--font); font-variant-numeric: tabular-nums; user-select: none; }
.hs-grid th { position: sticky; top: 0; background: var(--surface-2); color: var(--muted); font-weight: 500; font-size: 11.5px; border-right: 1px solid var(--border); border-bottom: 1px solid var(--border); padding: 3px 4px; z-index: 2; }
.hs-grid tbody th { position: sticky; left: 0; z-index: 1; text-align: center; }
.hs-grid th.hs-corner { left: 0; z-index: 3; }
.hs-grid th.on { background: color-mix(in srgb, #1d6f42 18%, var(--surface-2)); color: var(--text); }
.hs-grid td { border-right: 1px solid var(--border); border-bottom: 1px solid var(--border); padding: 4px 6px; height: 25px; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; position: relative; cursor: cell; color: var(--text); }
.hs-grid td.n { text-align: right; }
.hs-grid td.in { background: var(--in); }
.hs-grid td.out { background: var(--out); font-weight: 700; }
.hs-grid td.hdr { background: var(--surface-2); font-weight: 700; }
.hs-grid td.tot { font-weight: 700; border-top: 2px solid var(--border-2); }
.hs-grid td.note { color: var(--muted); font-style: italic; }
.hs-grid td.err { color: var(--bad); }
.hs-grid td.sel { background-image: linear-gradient(rgba(29,111,66,.13), rgba(29,111,66,.13)); }
.hs-grid td.cur { outline: 2px solid #1d6f42; outline-offset: -2px; z-index: 1; }
.hs-grid td.prec { box-shadow: inset 0 0 0 2px var(--s2); }
.hs-grid td.filltgt { box-shadow: inset 0 0 0 1px #1d6f42; background-image: linear-gradient(rgba(29,111,66,.08), rgba(29,111,66,.08)); }
.hs-fill { position: absolute; right: -1px; bottom: -1px; width: 7px; height: 7px; background: #1d6f42; border: 1px solid var(--surface); cursor: crosshair; z-index: 3; }
.hs-editor { position: absolute; inset: 0; width: 100%; border: 2px solid #1d6f42; font: 13px var(--mono); padding: 2px 5px; background: var(--surface); color: var(--text); outline: none; z-index: 4; }
.hs-status { font-size: 12px; color: var(--muted); padding: 4px 10px; border-top: 1px solid var(--border); background: var(--surface-2); min-height: 24px; }
.hs-status .err { color: var(--bad); }
.prec-key { color: var(--s2); font-weight: 700; }
.hs-help { font-size: 12.5px; padding: 8px 12px; background: var(--surface-2); border-top: 1px solid var(--border); color: var(--text-2); }
.hs-charts { padding: 0 10px; }
.hs-chart { margin: 8px 0; }
.tpl-row { margin: 10px 0; }

/* ---------------- charts */
.hbc { position: relative; }
.hbc svg { display: block; overflow: visible; }
.hbc-title { font: 600 13px var(--font); fill: var(--text); }
.hbc-grid { stroke: var(--border); stroke-width: 1; }
.hbc-axis { stroke: var(--border-2); }
.hbc-zero { stroke: var(--muted); stroke-width: 1; }
.hbc-tick { font: 11px var(--font); fill: var(--muted); font-variant-numeric: tabular-nums; }
.hbc-label { font: 12px var(--font); fill: var(--text-2); }
.hbc-ref { stroke: var(--muted); stroke-dasharray: 4 4; }
.hbc-reflabel { font: 11px var(--font); fill: var(--text-2); }
.hbc-cross { stroke: var(--muted); stroke-width: 1; }
.hbc-pt { fill: var(--text); stroke: var(--surface); stroke-width: 2; }
.hbc-ptlabel { font: 600 11.5px var(--font); fill: var(--text); paint-order: stroke; stroke: var(--surface); stroke-width: 3; }
.hbc-legend { display: flex; flex-wrap: wrap; gap: 4px 14px; font-size: 12.5px; color: var(--text-2); padding: 4px 0 0 56px; }
.hbc-legend i, .hbc-tip i { display: inline-block; width: 10px; height: 10px; border-radius: 3px; margin-right: 5px; vertical-align: -1px; }
.hbc-tip { position: absolute; pointer-events: none; background: var(--surface); border: 1px solid var(--border-2); border-radius: 8px; padding: 6px 10px; font-size: 12px; box-shadow: var(--shadow); min-width: 140px; z-index: 5; color: var(--text); }
.hbc-empty { color: var(--muted); padding: 20px; text-align: center; }

/* ---------------- tools */
.tool { border: 1px solid var(--border-2); border-radius: 12px; background: var(--surface); margin: 18px 0; box-shadow: var(--shadow); overflow: hidden; }
.tool-head { display: flex; align-items: center; gap: 12px; padding: 10px 14px; background: linear-gradient(90deg, var(--accent-soft), var(--surface)); border-bottom: 1px solid var(--border); }
.tool-icon { font-size: 22px; }
.tool-title { font-weight: 700; }
.tool-sub { font-size: 12.5px; color: var(--text-2); }
.tool-pop { margin-left: auto; color: var(--muted); font-size: 18px; }
.tool-body { padding: 14px 16px; }
.tool-page .tool { margin-top: 0; }
.t-grid { display: grid; grid-template-columns: minmax(220px, 280px) 1fr; gap: 18px; }
.t-controls { display: flex; flex-direction: column; gap: 9px; }
.t-out { min-width: 0; }
.ctl { display: grid; grid-template-columns: 1fr auto; gap: 2px 8px; align-items: center; font-size: 13px; }
.ctl label { color: var(--text-2); }
.ctl .v { font: 600 12.5px var(--mono); color: var(--text); text-align: right; min-width: 64px; }
.ctl input[type=range] { grid-column: 1 / -1; width: 100%; accent-color: var(--accent); }
.ctl input[type=number], .ctl select, .t-in { width: 100%; padding: 5px 8px; border: 1px solid var(--border-2); border-radius: 7px; font: 13px var(--mono); background: var(--in); color: var(--text); }
.ctl.inline { grid-template-columns: 1fr 110px; }
.kpis { display: grid; grid-template-columns: repeat(auto-fill, minmax(118px, 1fr)); gap: 8px; margin-bottom: 12px; }
.kpi { background: var(--surface-2); border-radius: 9px; padding: 8px 10px; border: 1px solid var(--border); }
.kpi span { display: block; font-size: 11.5px; color: var(--muted); }
.kpi b { font: 700 1.15rem var(--mono); font-variant-numeric: tabular-nums; }
.kpi.hi { background: var(--out); border-color: color-mix(in srgb, var(--good) 35%, transparent); }
.kpi.bad b { color: var(--bad); } .kpi.good b { color: var(--good); }
.t-note { font-size: 13px; color: var(--text-2); background: var(--surface-2); border-radius: 8px; padding: 8px 12px; margin-top: 10px; }
.t-note.warn { background: var(--warn-soft); } .t-note.good { background: var(--good-soft); } .t-note.bad { background: var(--bad-soft); }
.t-table { border-collapse: collapse; width: 100%; font: 12.5px var(--mono); font-variant-numeric: tabular-nums; }
.t-table th, .t-table td { border-bottom: 1px solid var(--border); padding: 4px 8px; text-align: right; }
.t-table th { background: var(--surface-2); font-family: var(--font); font-weight: 600; color: var(--text-2); position: sticky; top: 0; }
.t-table td:first-child, .t-table th:first-child { text-align: left; }
.t-table tr.call td { background: var(--bad-soft); }
.t-scroll { max-height: 320px; overflow: auto; border: 1px solid var(--border); border-radius: 8px; }
.t-row { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; margin: 8px 0; }
.legs { display: flex; flex-direction: column; gap: 6px; }
.leg { display: grid; grid-template-columns: 70px 80px 1fr 1fr 28px; gap: 5px; align-items: center; }
.leg select, .leg input { padding: 4px 6px; border: 1px solid var(--border-2); border-radius: 6px; font: 12.5px var(--mono); background: var(--in); color: var(--text); width: 100%; }
.leg button { border: 0; background: none; color: var(--bad); cursor: pointer; font-size: 16px; }
.leg-h { font-size: 11px; color: var(--muted); }
.tree-wrap { overflow: auto; border: 1px solid var(--border); border-radius: 8px; background: var(--surface); max-height: 520px; }
.tree-wrap svg text { font: 10.5px var(--mono); fill: var(--text); }
.tree-node rect { fill: var(--surface-2); stroke: var(--border-2); }
.tree-node.ex rect { fill: var(--warn-soft); stroke: var(--warn); }
.tree-edge { stroke: var(--border-2); }
.steps-list { font: 12.5px var(--mono); background: var(--surface-2); border-radius: 8px; padding: 8px 12px; max-height: 260px; overflow: auto; }
.steps-list div { padding: 1px 0; }

/* ---------------- practice / flashcards / misc pages */
.filter-row { display: flex; gap: 8px; flex-wrap: wrap; margin: 12px 0 18px; align-items: center; }
.filter-row input, .filter-row select, .fc-bar select { padding: 8px 12px; border: 1px solid var(--border-2); border-radius: 9px; font: inherit; font-size: 14px; background: var(--surface); color: var(--text); }
.filter-row input { flex: 1; min-width: 200px; }
.gloss td { font-size: 14px; } .zh-cell { white-space: nowrap; }
.s-res { display: block; padding: 12px 16px; border: 1px solid var(--border); border-radius: 10px; background: var(--surface); margin-bottom: 8px; color: var(--text); }
.s-res:hover { border-color: var(--accent); text-decoration: none; }
.s-t { font-weight: 600; } .s-t small { color: var(--muted); font-weight: 400; margin-left: 6px; }
.s-snip { font-size: 13.5px; color: var(--text-2); margin-top: 4px; }
.note-card { margin-bottom: 10px; }
.note-card pre { white-space: pre-wrap; font: 14px/1.6 var(--font); margin: 8px 0 0; }
.fc-bar { display: flex; gap: 10px; flex-wrap: wrap; align-items: center; margin: 10px 0; }
.fc-stats { display: flex; gap: 18px; align-items: center; color: var(--text-2); font-size: 13.5px; margin: 10px 0 20px; flex-wrap: wrap; }
.flashcard { max-width: 640px; min-height: 280px; margin: 0 auto; perspective: 1200px; position: relative; cursor: pointer; outline: none; }
.flashcard > div { position: absolute; inset: 0; backface-visibility: hidden; transition: transform .45s; background: var(--surface); border: 1px solid var(--border-2); border-radius: 16px; box-shadow: var(--shadow); padding: 30px; display: flex; flex-direction: column; align-items: center; justify-content: center; text-align: center; }
.fc-back { transform: rotateY(180deg); }
.flashcard.flipped .fc-front { transform: rotateY(180deg); }
.flashcard.flipped .fc-back { transform: rotateY(360deg); }
.fc-term { font: 700 1.8rem var(--serif); }
.fc-term.zh { font-family: var(--font); }
.fc-zh { font-size: 1.3rem; color: var(--accent-ink); margin: 4px 0 12px; }
.fc-def { color: var(--text-2); font-size: 1.02rem; max-width: 50ch; }
.fc-hint { position: absolute; bottom: 14px; font-size: 12px; color: var(--muted); }
.fc-src { position: absolute; bottom: 14px; right: 18px; font-size: 12.5px; }
.fc-actions { display: flex; justify-content: center; gap: 10px; margin-top: 22px; }
.fc-actions kbd { font-size: 11px; }
.pr-card { max-width: 760px; }
.pr-q { font-size: 1.05rem; margin: 6px 0 12px; }
.pr-steps { margin-top: 10px; }
.pr-steps li { margin: 4px 0; }
.about h3 { margin-top: 0; }

/* =================================================================
   Apple-style visual layer — same markup and content, new look
   ================================================================= */
body { font-size: 16.5px; line-height: 1.6; letter-spacing: -.011em; text-rendering: optimizeLegibility; }
:root[data-lang="zh"] body { letter-spacing: 0; }
h1, h2, h3, h4 { letter-spacing: -.02em; }
h1 { font-weight: 700; letter-spacing: -.028em; line-height: 1.08; }
code, kbd { background: var(--fill); border: 0; border-radius: 6px; padding: .05em .4em; }
kbd { box-shadow: inset 0 -1px 0 var(--border-2); }
#scrollbar { height: 2px; }
::selection { background: color-mix(in srgb, var(--accent) 24%, transparent); }

/* top bar: translucent glass, quiet links */
.top { background: var(--nav-bg); backdrop-filter: saturate(180%) blur(20px); -webkit-backdrop-filter: saturate(180%) blur(20px); border-bottom: 1px solid var(--nav-line); gap: 8px; padding: 0 16px; }
.brand { font-weight: 600; font-size: 15px; letter-spacing: -.015em; gap: 9px; }
.brand small { font-size: 11px; letter-spacing: 0; }
.brand-mark { width: 28px; height: 28px; border-radius: 7px; background: var(--text); color: var(--surface); font: 600 17px var(--font); }
.topnav { gap: 0; margin-left: 10px; }
.topnav a { color: var(--nav-ink); font-size: 12.5px; letter-spacing: -.005em; padding: 6px 10px; border-radius: 980px; opacity: .88; transition: color .2s, opacity .2s, background .2s; }
.topnav a:hover { background: none; color: var(--text); opacity: 1; }
.topnav a.on { background: var(--fill); color: var(--text); opacity: 1; font-weight: 600; }
.search input { width: 200px; border: 0; border-radius: 980px; background: var(--fill); padding: 6px 14px; font-size: 13px; letter-spacing: -.005em; }
.search input:focus { outline: 0; box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent) 35%, transparent); background: var(--surface); }
.lang-sel { border: 0; border-radius: 980px; background: var(--fill); font: 500 12px var(--font); padding: 6px 10px; }
.icon-btn { color: var(--nav-ink); font-size: 16px; width: 34px; height: 34px; border-radius: 50%; border: 0; transition: background .2s, color .2s; }
.icon-btn:hover { background: var(--fill); color: var(--text); }

/* sidebar: frosted list, Apple-style selection */
.side { background: var(--side-bg); backdrop-filter: saturate(180%) blur(20px); -webkit-backdrop-filter: saturate(180%) blur(20px); border-right: 1px solid var(--nav-line); padding: 12px 10px 48px; }
.toc-part-name { font-size: 11.5px; letter-spacing: .02em; padding: 14px 10px 6px; }
.toc-ch { border-radius: 10px; padding: 7px 10px; font-size: 13.5px; transition: background .15s; }
.toc-ch:hover { background: var(--fill); }
.toc-ch.active { background: var(--accent-soft); color: var(--accent-ink); }

/* main column */
.main { padding-top: calc(var(--top) + 40px); }
.rightnav a { border-left-width: 2px; padding: 4px 10px; }
.rn-title { letter-spacing: .02em; }

/* buttons, chips, segmented controls */
.btn { border: 0; border-radius: 980px; background: var(--btn); padding: 9px 20px; font: 400 15px var(--font); letter-spacing: -.01em; transition: background .2s, color .2s, box-shadow .2s; }
.btn:hover { filter: none; background: var(--btn-hover); }
.btn.ghost { background: transparent; color: var(--accent-ink); box-shadow: inset 0 0 0 1px var(--accent-ink); }
.btn.ghost:hover { background: var(--accent-ink); color: #fff; }
.btn.sm { padding: 5px 14px; font-size: 13px; border-radius: 980px; }
.chip { border: 0; background: var(--fill); color: var(--text); padding: 5px 13px; }
.chip.on { background: var(--text); color: var(--surface); }
.seg { border: 0; background: var(--fill); border-radius: 10px; padding: 2px; gap: 2px; }
.seg button { border: 0 !important; background: transparent; border-radius: 8px; color: var(--text); padding: 5px 13px; transition: background .2s, box-shadow .2s; }
.seg button.on { background: var(--seg-on); color: var(--text); box-shadow: 0 1px 3px rgba(0,0,0,.12), 0 0 0 .5px rgba(0,0,0,.04); }

/* cards and tiles: borderless, rounded, soft shadow */
.card, .sec, .tool, .ch-card, .tool-card, .stats > div, .pn, .s-res, .terms-box, .fcard { border: 0; border-radius: var(--radius); box-shadow: var(--shadow); }
.card { padding: 22px 24px; }
.formulas-sec .fcard { box-shadow: none; }
.ch-card, .tool-card, .pn, .s-res { transition: transform .25s cubic-bezier(.2,.8,.2,1), box-shadow .25s; }
.ch-card:hover, .tool-card:hover, .pn:hover, .s-res:hover { transform: translateY(-3px); box-shadow: var(--shadow-hover); }
.ch-card { padding: 18px; }
.cc-num { font: 700 1.6rem var(--font); letter-spacing: -.03em; color: var(--text); }
.cc-title { letter-spacing: -.015em; }
.tool-card { padding: 18px; }
.tool-card:hover { border: 0; }
.tc-icon { font-size: 24px; }
.pn { padding: 18px 22px; }
.pn:hover { color: var(--accent-ink); }

/* home */
.hero { border-bottom: 0; padding: 20px 0 36px; gap: 36px; align-items: center; }
.hero h1 { font-size: clamp(2.5rem, 4.4vw, 3.6rem); line-height: 1.05; letter-spacing: -.035em; }
.kicker, .ch-kicker { text-transform: none; letter-spacing: -.01em; font-size: 15px; font-weight: 600; color: var(--eyebrow); }
.hero-actions { gap: 12px; margin-top: 24px; }
.hero > * { min-width: 0; }
.hero-actions .btn { max-width: 100%; white-space: normal; text-align: center; justify-content: center; line-height: 1.35; }
.stats { gap: 12px; }
.stats > div { padding: 16px 18px; }
.stats b { font-size: 2.1rem; font-weight: 700; letter-spacing: -.03em; line-height: 1.1; }
.stats span { font-size: 13px; }
.part-h { font-size: 1.75rem; font-weight: 700; letter-spacing: -.025em; margin: 52px 0 16px; }
.plan-bar { border-radius: 980px; height: 10px; }
.plan-bar i { border-radius: 980px; }
.plan-form input { border-radius: 10px; }

/* chapter */
.ch-head { border-bottom: 0; padding-bottom: 8px; margin-bottom: 12px; }
.ch-head h1 { font-size: clamp(2.2rem, 3.8vw, 3.1rem); line-height: 1.07; letter-spacing: -.035em; margin-top: .2em; }
.ch-zh { font-size: 1.2rem; letter-spacing: -.01em; }
.ch-progress .bar, .fc-stats .bar { border-radius: 980px; background: var(--surface-3); }
.intro { font-size: 1.1rem; line-height: 1.6; letter-spacing: -.015em; }
.sec { border-radius: 24px; padding: 10px 36px 24px; margin: 26px 0; }
.sec.is-read { border-left: 0; box-shadow: var(--shadow), inset 0 0 0 1.5px color-mix(in srgb, var(--good) 55%, transparent); }
.sec h2 { font-size: 1.65rem; font-weight: 700; letter-spacing: -.025em; margin: 22px 0 12px; }
.sec-id { border-radius: 980px; padding: 3px 10px; font-size: .74rem; letter-spacing: 0; }
.sec-foot { border-top: 1px solid var(--border); padding-top: 14px; }
.zh, .en { border: 0; background: var(--fill); border-radius: 6px; }
:root[data-lang="bi"] .zh { background: var(--accent-soft); }
.formula { border: 0; border-radius: 14px; padding: 8px 18px; }
.callout { border-radius: 14px; padding: 14px 18px; }
.callout, .callout.tip, .callout.warn, .callout.example, .callout.q { border-color: transparent; }
.callout.def, .callout.key, .callout.exam { border-top-color: transparent; border-right-color: transparent; border-bottom-color: transparent; border-left-width: 3px; }
.callout.exam > b:first-child { letter-spacing: .02em; }
.tbl { font-size: 14.5px; }
.tbl th, .tbl td { border: 0; border-bottom: 1px solid var(--border); padding: 9px 12px; }
.tbl thead th { background: transparent; color: var(--muted); font-size: 13px; border-bottom-color: var(--border-2); }
.mynote textarea { border-radius: 14px; border-color: var(--border); }
.copy-tex { border-radius: 8px; }

/* quizzes and tests */
.quiz { border: 0; border-radius: 18px; padding: 6px 20px 14px; }
.q-n { background: var(--surface-3); }
.q-opt { border-radius: 12px; border-color: transparent; background: var(--surface); box-shadow: 0 0 0 1px var(--border); transition: box-shadow .15s, background .15s; }
.q-opt:hover { border-color: transparent; box-shadow: 0 0 0 1px var(--border-2); }
.q-opt:has(input:checked) { border-color: transparent; box-shadow: 0 0 0 2px var(--accent); }
.q-opt.is-answer { border-color: transparent; box-shadow: 0 0 0 2px var(--good); }
.q-num input { border-radius: 10px; }
.q-num input:focus { outline: 0; box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent) 35%, transparent); }
.q-feedback { border-radius: 12px; }
.exam-sec { border: 0; box-shadow: var(--shadow), inset 0 0 0 2px var(--accent); }
.exam-head h3 { font-weight: 700; letter-spacing: -.025em; }
.score-big { font-family: var(--font); letter-spacing: -.03em; }

/* spreadsheets: Numbers-like, accent selection */
.hs-wrap { border: 0; border-radius: 16px; box-shadow: var(--shadow), 0 0 0 1px var(--border); }
.hs-head { background: var(--surface); color: var(--text); border-bottom: 1px solid var(--border); padding: 7px 10px 7px 14px; }
.hs-logo { background: var(--s3); color: #fff; border-radius: 6px; }
.hs-tools button { background: var(--fill); color: var(--text); border: 0; border-radius: 980px; padding: 3px 11px; transition: background .15s; }
.hs-tools button:hover { background: var(--surface-3); }
.hs-tools button.on { background: var(--accent); color: #fff; }
.hs-name:focus, .hs-formula:focus { outline-color: var(--accent); }
.hs-grid th.on { background: color-mix(in srgb, var(--accent) 16%, var(--surface-2)); }
.hs-grid td.sel { background-image: linear-gradient(color-mix(in srgb, var(--accent) 13%, transparent), color-mix(in srgb, var(--accent) 13%, transparent)); }
.hs-grid td.cur { outline-color: var(--accent); }
.hs-grid td.filltgt { box-shadow: inset 0 0 0 1px var(--accent); background-image: linear-gradient(color-mix(in srgb, var(--accent) 8%, transparent), color-mix(in srgb, var(--accent) 8%, transparent)); }
.hs-fill { background: var(--accent); }
.hs-editor { border-color: var(--accent); }

/* charts */
.hbc-tip { border: 0; border-radius: 12px; box-shadow: 0 0 0 1px var(--border), 0 8px 24px rgba(0,0,0,.12); }

/* tools */
.tool { border-radius: 22px; }
.tool-head { background: var(--surface); padding: 14px 18px; }
.tool-title { font-weight: 600; letter-spacing: -.015em; }
.tool-body { padding: 18px 20px; }
.ctl input[type=number], .ctl select, .t-in, .leg select, .leg input { border-radius: 8px; border-color: var(--border); }
.kpi { border: 0; border-radius: 14px; padding: 10px 12px; }
.kpi b { font: 600 1.2rem var(--font); letter-spacing: -.02em; font-variant-numeric: tabular-nums; }
.kpi.hi { box-shadow: inset 0 0 0 1px color-mix(in srgb, var(--good) 35%, transparent); }
.t-note { border-radius: 12px; }
.t-scroll, .tree-wrap { border-radius: 14px; }
.steps-list { border-radius: 12px; }

/* practice, flashcards, search, misc */
.filter-row input, .filter-row select, .fc-bar select { border-radius: 980px; border: 0; background: var(--fill); padding: 8px 16px; }
.filter-row input:focus { outline: 0; box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent) 35%, transparent); }
.flashcard > div { border: 0; border-radius: 24px; box-shadow: var(--shadow-hover); }
.fc-term { font-family: var(--font); letter-spacing: -.03em; }

/* ---------------- responsive */
@media (max-width: 1100px) {
  .topnav { display: none; }
  .hero { grid-template-columns: 1fr; }
}
@media (max-width: 900px) {
  #menu-btn { display: block; }
  .side { transform: translateX(-100%); transition: transform .2s; box-shadow: var(--shadow); }
  body.nav-open .side { transform: none; }
  body.nav-open #scrim { display: block; position: fixed; inset: var(--top) 0 0 0; background: rgba(0,0,0,.3); z-index: 39; }
  .main { margin-left: 0; padding: calc(var(--top) + 18px) 16px 100px; }
  .t-grid { grid-template-columns: 1fr; }
  .home-grid { grid-template-columns: 1fr; }
  .search input { width: 140px; }
  .sec { padding: 4px 16px 14px; }
  .brand small { display: none; }
  .q-opts, .q-num, .q-actions, .q-feedback { margin-left: 0; }
}
@media (max-width: 520px) {
  .top { gap: 4px; padding: 0 10px; }
  .top > * { flex-shrink: 0; }
  .top .icon-btn { width: 32px; height: 32px; }
  .lang-sel { padding: 6px 8px; max-width: 92px; }
  .search { display: none; }
  h1, .ch-head h1 { font-size: 1.7rem; }
  .hero h1 { font-size: 1.9rem; }
  .prevnext { grid-template-columns: 1fr; }
  .leg { grid-template-columns: 60px 70px 1fr 1fr 24px; }
}
@media print {
  .top, .side, .rightnav, .sec-foot, .quiz, .exam-sec, .prevnext, .hs-head .hs-tools { display: none !important; }
  .main { margin: 0; padding: 0; max-width: none; }
  .sec { box-shadow: none; break-inside: avoid-page; }
}