/* ---------- Tokens ---------- */
:root {
  --fs: 17px;
  --bg: #f5f3ec;
  --bg-grid: rgba(15, 118, 110, .07);
  --surface: #fffdf8;
  --surface-2: #eeebe1;
  --line: #ddd8ca;
  --ink: #1b2130;
  --ink-2: #4a5263;
  --muted: #7a8090;
  --accent: #0f766e;
  --accent-ink: #ffffff;
  --accent-soft: rgba(15, 118, 110, .1);
  --warm: #c2410c;
  --warm-soft: rgba(194, 65, 12, .1);
  --good: #15803d;
  --bad: #b91c1c;
  --code-bg: #1b2130;
  --code-ink: #e6e9ef;
  --shadow: 0 1px 2px rgba(27, 33, 48, .06), 0 8px 24px rgba(27, 33, 48, .06);
  --radius: 14px;
  --top: 64px;
  --side: 300px;
  --font-body: 'Inter', system-ui, -apple-system, 'Segoe UI', sans-serif;
  --font-head: 'Space Grotesk', 'Inter', system-ui, sans-serif;
  --font-mono: 'JetBrains Mono', ui-monospace, Consolas, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0c1220; --bg-grid: rgba(45, 212, 191, .05); --surface: #121a2b; --surface-2: #19233a;
    --line: #26314a; --ink: #e6e9ef; --ink-2: #b4bccb; --muted: #7d879b;
    --accent: #2dd4bf; --accent-ink: #062320; --accent-soft: rgba(45, 212, 191, .12);
    --warm: #fb923c; --warm-soft: rgba(251, 146, 60, .12); --good: #4ade80; --bad: #f87171;
    --code-bg: #080d18; --code-ink: #e6e9ef; --shadow: 0 1px 2px rgba(0,0,0,.3), 0 8px 24px rgba(0,0,0,.25);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #0c1220; --bg-grid: rgba(45, 212, 191, .05); --surface: #121a2b; --surface-2: #19233a;
  --line: #26314a; --ink: #e6e9ef; --ink-2: #b4bccb; --muted: #7d879b;
  --accent: #2dd4bf; --accent-ink: #062320; --accent-soft: rgba(45, 212, 191, .12);
  --warm: #fb923c; --warm-soft: rgba(251, 146, 60, .12); --good: #4ade80; --bad: #f87171;
  --code-bg: #080d18; --code-ink: #e6e9ef; --shadow: 0 1px 2px rgba(0,0,0,.3), 0 8px 24px rgba(0,0,0,.25);
  color-scheme: dark;
}

/* ---------- Base ---------- */
*, *::before, *::after { box-sizing: border-box; }
html { font-size: var(--fs); scroll-padding-top: calc(var(--top) + 16px); -webkit-text-size-adjust: 100%; }
body {
  margin: 0; background: var(--bg); color: var(--ink); font-family: var(--font-body);
  line-height: 1.65; -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); text-decoration-thickness: 1px; text-underline-offset: 3px; }
button { font: inherit; color: inherit; cursor: pointer; }
kbd { font-family: var(--font-mono); font-size: .75em; padding: 1px 6px; border: 1px solid var(--line); border-bottom-width: 2px; border-radius: 5px; background: var(--surface); color: var(--ink-2); }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 6px; }
svg { fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; stroke-linejoin: round; }
.muted { color: var(--muted); }

.read-progress { position: fixed; top: 0; left: 0; height: 3px; width: 0; background: linear-gradient(90deg, var(--accent), var(--warm)); z-index: 60; transition: width .1s linear; }

/* ---------- Top bar ---------- */
.topbar {
  position: sticky; top: 0; z-index: 50; height: var(--top); display: flex; align-items: center; gap: 14px;
  padding: 0 18px; background: color-mix(in srgb, var(--bg) 85%, transparent); backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--line);
}
.icon-btn { display: inline-grid; place-items: center; width: 38px; height: 38px; border-radius: 10px; border: 1px solid transparent; background: none; color: var(--ink-2); }
.icon-btn:hover { background: var(--surface-2); color: var(--ink); }
.icon-btn svg { width: 20px; height: 20px; }
.aa { font-family: var(--font-head); font-weight: 700; font-size: 1.05rem; }
.aa.small { font-size: .78rem; }
.menu-toggle { display: none; }
.brand { display: flex; align-items: center; gap: 10px; color: var(--ink); text-decoration: none; min-width: 0; }
.brand-mark { display: grid; place-items: center; width: 36px; height: 36px; border-radius: 10px; background: var(--ink); color: var(--accent); flex: none; }
.brand-mark svg { width: 24px; height: 24px; stroke-width: 2.2; }
.brand-text { display: flex; flex-direction: column; line-height: 1.15; min-width: 0; }
.brand-text strong { font-family: var(--font-head); font-size: 1.02rem; white-space: nowrap; }
.brand-text em { font-style: normal; color: var(--accent); }
.brand-text small { color: var(--muted); font-size: .72rem; }
.search-wrap { position: relative; flex: 1; max-width: 560px; margin: 0 auto; }
.search-wrap input {
  width: 100%; height: 40px; padding: 0 76px 0 38px; border-radius: 12px; border: 1px solid var(--line);
  background: var(--surface); color: var(--ink); font: inherit; font-size: .9rem;
}
.search-wrap input:focus { outline: none; border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.search-icon { position: absolute; left: 12px; top: 11px; width: 18px; height: 18px; color: var(--muted); }
.kbd-hint { position: absolute; right: 10px; top: 9px; }
.search-results {
  position: absolute; top: 46px; left: 0; right: 0; max-height: min(70vh, 560px); overflow: auto; background: var(--surface);
  border: 1px solid var(--line); border-radius: 14px; box-shadow: var(--shadow); padding: 6px;
}
.sr-item { display: block; padding: 10px 12px; border-radius: 10px; color: var(--ink); text-decoration: none; }
.sr-item[aria-selected="true"], .sr-item:hover { background: var(--accent-soft); }
.sr-item b { display: block; font-size: .92rem; }
.sr-item small { display: block; color: var(--muted); font-size: .76rem; }
.sr-item p { margin: 4px 0 0; color: var(--ink-2); font-size: .82rem; line-height: 1.45; }
.sr-item mark { background: var(--warm-soft); color: var(--warm); border-radius: 3px; padding: 0 2px; }
.sr-empty { padding: 14px; color: var(--muted); font-size: .88rem; }
.top-actions { display: flex; gap: 2px; }
:root .i-moon { display: none; }
:root[data-theme="dark"] .i-sun { display: none; }
:root[data-theme="dark"] .i-moon { display: block; }
@media (prefers-color-scheme: dark) { :root:not([data-theme="light"]) .i-sun { display: none; } :root:not([data-theme="light"]) .i-moon { display: block; } }

/* ---------- Layout ---------- */
.layout { display: grid; grid-template-columns: var(--side) minmax(0, 1fr); }
.sidebar {
  position: sticky; top: var(--top); height: calc(100vh - var(--top)); overflow-y: auto; border-right: 1px solid var(--line);
  background: var(--surface); display: flex; flex-direction: column; overscroll-behavior: contain;
}
.side-progress { display: flex; align-items: center; gap: 12px; padding: 18px 18px 14px; border-bottom: 1px solid var(--line); }
.side-progress strong { display: block; font-size: .9rem; }
.side-progress small { color: var(--muted); font-size: .78rem; }
.ring {
  --p: 0; width: 46px; height: 46px; border-radius: 50%; flex: none; display: grid; place-items: center;
  background: conic-gradient(var(--accent) calc(var(--p) * 1%), var(--surface-2) 0);
}
.ring span { display: grid; place-items: center; width: 36px; height: 36px; border-radius: 50%; background: var(--surface); font-size: .7rem; font-weight: 700; }
.nav { padding: 8px 10px 16px; flex: 1; }
.nav-part { margin: 16px 10px 6px; font-size: .68rem; letter-spacing: .12em; text-transform: uppercase; color: var(--muted); font-weight: 700; }
.nav-ch > button {
  width: 100%; display: flex; align-items: center; gap: 10px; padding: 8px 10px; border: 0; border-radius: 10px; background: none;
  text-align: left; font-size: .88rem; font-weight: 600; color: var(--ink);
}
.nav-ch > button:hover { background: var(--surface-2); }
.nav-ch .ch-icon { width: 26px; height: 26px; display: grid; place-items: center; border-radius: 8px; background: var(--accent-soft); color: var(--accent); font-size: .85rem; flex: none; }
.nav-ch .ch-name { flex: 1; line-height: 1.3; }
.nav-ch .ch-count { font-size: .7rem; color: var(--muted); font-weight: 500; }
.nav-ch .chev { width: 14px; height: 14px; color: var(--muted); transition: transform .15s; flex: none; }
.nav-ch.open .chev { transform: rotate(90deg); }
.nav-ch ul { list-style: none; margin: 2px 0 8px; padding: 0 0 0 22px; display: none; border-left: 1px solid var(--line); margin-left: 22px; }
.nav-ch.open ul { display: block; }
.nav-ch li a {
  display: flex; gap: 8px; align-items: flex-start; padding: 6px 10px; border-radius: 8px; font-size: .82rem; line-height: 1.4;
  color: var(--ink-2); text-decoration: none;
}
.nav-ch li a:hover { background: var(--surface-2); color: var(--ink); }
.nav-ch li a.active { background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.dot { width: 14px; height: 14px; border-radius: 50%; border: 1.5px solid var(--line); flex: none; margin-top: 2px; display: grid; place-items: center; }
.dot.done { background: var(--accent); border-color: var(--accent); }
.dot.done::after { content: ''; width: 6px; height: 3px; border: solid var(--accent-ink); border-width: 0 0 1.6px 1.6px; transform: rotate(-45deg) translate(1px, -1px); }
.side-foot { padding: 12px 18px 18px; border-top: 1px solid var(--line); }
.ghost-btn { border: 1px solid var(--line); background: none; border-radius: 10px; padding: 7px 12px; font-size: .8rem; color: var(--ink-2); }
.ghost-btn:hover { border-color: var(--bad); color: var(--bad); }
.scrim { display: none; }

.main { min-width: 0; }
.view { padding: 32px 40px 48px; min-height: calc(100vh - var(--top) - 120px); }
.site-foot { padding: 24px 40px 40px; border-top: 1px solid var(--line); font-size: .82rem; color: var(--ink-2); }
.site-foot p { margin: 4px 0; }

/* ---------- Buttons ---------- */
.btn { display: inline-flex; align-items: center; gap: 8px; padding: 10px 18px; border-radius: 12px; border: 1px solid var(--line); background: var(--surface); color: var(--ink); font-weight: 600; font-size: .9rem; text-decoration: none; }
.btn:hover { border-color: var(--accent); color: var(--accent); }
.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }
.btn.primary:hover { filter: brightness(1.08); color: var(--accent-ink); }
.btn svg { width: 18px; height: 18px; }

/* ---------- Home ---------- */
.hero {
  position: relative; overflow: hidden; border-radius: 22px; padding: 48px 44px; margin-bottom: 36px; border: 1px solid var(--line);
  background-color: var(--surface);
  background-image: linear-gradient(var(--bg-grid) 1px, transparent 1px), linear-gradient(90deg, var(--bg-grid) 1px, transparent 1px);
  background-size: 28px 28px;
}
.hero .eyebrow { display: inline-block; font-size: .72rem; letter-spacing: .14em; text-transform: uppercase; font-weight: 700; color: var(--warm); background: var(--warm-soft); padding: 4px 10px; border-radius: 99px; }
.hero h1 { font-family: var(--font-head); font-size: clamp(2rem, 4.4vw, 3.1rem); line-height: 1.08; margin: 16px 0 12px; max-width: 18ch; letter-spacing: -.02em; }
.hero h1 span { color: var(--accent); }
.hero p { max-width: 60ch; color: var(--ink-2); margin: 0 0 24px; }
.hero-actions { display: flex; gap: 10px; flex-wrap: wrap; }
.stats { display: flex; gap: 28px; flex-wrap: wrap; margin-top: 32px; }
.stat b { display: block; font-family: var(--font-head); font-size: 1.6rem; line-height: 1; }
.stat small { color: var(--muted); font-size: .78rem; }
.part-head { display: flex; align-items: baseline; gap: 12px; margin: 34px 0 14px; }
.part-head h2 { font-family: var(--font-head); font-size: 1.2rem; margin: 0; }
.part-head span { color: var(--muted); font-size: .8rem; }
.cards { display: grid; grid-template-columns: repeat(auto-fill, minmax(280px, 1fr)); gap: 16px; }
.card {
  display: flex; flex-direction: column; gap: 8px; padding: 20px; border-radius: var(--radius); border: 1px solid var(--line);
  background: var(--surface); color: var(--ink); text-decoration: none; transition: transform .15s, border-color .15s, box-shadow .15s;
}
.card:hover { transform: translateY(-2px); border-color: var(--accent); box-shadow: var(--shadow); }
.card-top { display: flex; align-items: center; gap: 10px; }
.card .ch-icon { width: 34px; height: 34px; border-radius: 10px; display: grid; place-items: center; background: var(--accent-soft); color: var(--accent); font-size: 1rem; }
.card .num { font-family: var(--font-mono); font-size: .72rem; color: var(--muted); }
.card h3 { font-family: var(--font-head); font-size: 1.05rem; margin: 2px 0 0; line-height: 1.3; }
.card p { margin: 0; color: var(--ink-2); font-size: .85rem; line-height: 1.5; flex: 1; }
.bar { height: 6px; border-radius: 99px; background: var(--surface-2); overflow: hidden; }
.bar i { display: block; height: 100%; background: var(--accent); border-radius: 99px; }
.card-foot { display: flex; justify-content: space-between; font-size: .74rem; color: var(--muted); }

/* ---------- Chapter page ---------- */
.crumbs { font-size: .8rem; color: var(--muted); margin-bottom: 10px; display: flex; gap: 6px; flex-wrap: wrap; }
.crumbs a { color: var(--muted); text-decoration: none; }
.crumbs a:hover { color: var(--accent); }
.ch-hero { padding: 28px 30px; border-radius: 18px; border: 1px solid var(--line); background: var(--surface); margin-bottom: 22px; }
.ch-hero h1 { font-family: var(--font-head); font-size: clamp(1.6rem, 3vw, 2.2rem); margin: 6px 0 8px; line-height: 1.15; }
.ch-hero p { color: var(--ink-2); margin: 0 0 16px; max-width: 65ch; }
.lesson-list { list-style: none; padding: 0; margin: 0; display: grid; gap: 8px; max-width: 860px; }
.lesson-list a {
  display: flex; align-items: center; gap: 14px; padding: 14px 16px; border-radius: 12px; border: 1px solid var(--line);
  background: var(--surface); color: var(--ink); text-decoration: none;
}
.lesson-list a:hover { border-color: var(--accent); }
.lesson-list .idx { font-family: var(--font-mono); font-size: .78rem; color: var(--muted); width: 26px; }
.lesson-list .t { flex: 1; font-weight: 500; font-size: .95rem; }
.tag { font-size: .66rem; letter-spacing: .06em; text-transform: uppercase; font-weight: 700; padding: 3px 8px; border-radius: 99px; background: var(--surface-2); color: var(--ink-2); white-space: nowrap; }
.tag.qa, .tag.revision { background: var(--warm-soft); color: var(--warm); }
.tag.case, .tag.lld, .tag.lab { background: var(--accent-soft); color: var(--accent); }

/* ---------- Lesson ---------- */
.lesson-wrap { display: grid; grid-template-columns: minmax(0, 760px) 220px; gap: 48px; justify-content: center; }
.lesson-head h1 { font-family: var(--font-head); font-size: clamp(1.7rem, 3.4vw, 2.4rem); line-height: 1.15; margin: 4px 0 10px; letter-spacing: -.015em; }
.meta { display: flex; gap: 14px; flex-wrap: wrap; align-items: center; color: var(--muted); font-size: .8rem; margin-bottom: 26px; }
.toc { position: sticky; top: calc(var(--top) + 28px); align-self: start; font-size: .8rem; max-height: calc(100vh - var(--top) - 56px); overflow: auto; }
.toc b { display: block; font-size: .68rem; letter-spacing: .12em; text-transform: uppercase; color: var(--muted); margin-bottom: 8px; }
.toc a { display: block; padding: 5px 0 5px 12px; border-left: 2px solid var(--line); color: var(--ink-2); text-decoration: none; line-height: 1.35; }
.toc a:hover { color: var(--ink); }
.toc a.on { border-color: var(--accent); color: var(--accent); font-weight: 600; }

.prose { font-size: 1rem; }
.prose > :first-child { margin-top: 0; }
.prose h2 { font-family: var(--font-head); font-size: 1.45rem; line-height: 1.25; margin: 2.2em 0 .6em; letter-spacing: -.01em; padding-top: .4em; border-top: 1px dashed var(--line); }
.prose h3 { font-family: var(--font-head); font-size: 1.12rem; margin: 1.7em 0 .5em; }
.prose h4 { font-size: 1rem; margin: 1.4em 0 .4em; }
.prose p, .prose ul, .prose ol { margin: 0 0 1em; }
.prose li { margin: .3em 0; }
.prose li::marker { color: var(--accent); }
.prose strong { color: var(--ink); }
.prose hr { border: 0; border-top: 1px solid var(--line); margin: 2em 0; }
.prose img { max-width: 100%; }
.prose :not(pre) > code { font-family: var(--font-mono); font-size: .86em; background: var(--surface-2); padding: .12em .4em; border-radius: 6px; color: var(--warm); }
.prose pre { position: relative; background: var(--code-bg); color: var(--code-ink); border-radius: 12px; padding: 16px 18px; overflow-x: auto; font-size: .82rem; line-height: 1.6; margin: 0 0 1.2em; }
.prose pre code { font-family: var(--font-mono); }
.copy-btn { position: absolute; top: 8px; right: 8px; font-size: .7rem; padding: 3px 8px; border-radius: 6px; border: 1px solid rgba(255,255,255,.18); background: rgba(255,255,255,.06); color: #cbd2de; opacity: 0; transition: opacity .15s; }
.prose pre:hover .copy-btn, .copy-btn:focus { opacity: 1; }
.table-wrap { overflow-x: auto; margin: 0 0 1.2em; border: 1px solid var(--line); border-radius: 12px; }
.prose table { border-collapse: collapse; width: 100%; font-size: .88rem; }
.prose th, .prose td { padding: 10px 12px; text-align: left; border-bottom: 1px solid var(--line); vertical-align: top; }
.prose th { background: var(--surface-2); font-weight: 600; white-space: nowrap; }
.prose tr:last-child td { border-bottom: 0; }
.prose blockquote { margin: 0 0 1.2em; padding: 12px 18px; border-left: 3px solid var(--line); color: var(--ink-2); background: var(--surface); border-radius: 0 12px 12px 0; }

.callout { margin: 0 0 1.3em; padding: 14px 18px 14px 16px; border-radius: 12px; border: 1px solid var(--line); background: var(--surface); border-left: 4px solid var(--accent); }
.callout > :last-child { margin-bottom: 0; }
.callout-title { display: flex; align-items: center; gap: 8px; font-size: .72rem; letter-spacing: .1em; text-transform: uppercase; font-weight: 800; margin-bottom: 6px; color: var(--accent); }
.callout.tip { border-left-color: var(--good); } .callout.tip .callout-title { color: var(--good); }
.callout.warning { border-left-color: var(--bad); } .callout.warning .callout-title { color: var(--bad); }
.callout.interview { border-left-color: var(--warm); background: var(--warm-soft); } .callout.interview .callout-title { color: var(--warm); }
.callout.example { border-left-color: var(--ink-2); } .callout.example .callout-title { color: var(--ink-2); }

.diagram { margin: 0 0 1.4em; padding: 18px; border: 1px solid var(--line); border-radius: 14px; background: var(--surface); overflow-x: auto; text-align: center; }
.diagram svg { max-width: 100%; height: auto; }
.diagram.loading { color: var(--muted); font-size: .85rem; }

.prose details { border: 1px solid var(--line); border-radius: 12px; background: var(--surface); margin: 0 0 10px; }
.prose details > summary { list-style: none; cursor: pointer; padding: 14px 44px 14px 16px; font-weight: 600; position: relative; }
.prose details > summary::-webkit-details-marker { display: none; }
.prose details > summary::after { content: '+'; position: absolute; right: 16px; top: 11px; font-size: 1.3rem; color: var(--accent); font-weight: 400; }
.prose details[open] > summary::after { content: '−'; }
.prose details[open] > summary { border-bottom: 1px dashed var(--line); }
.prose details > :not(summary) { margin-left: 16px; margin-right: 16px; }
.prose details > :nth-child(2) { margin-top: 12px; }
.prose details > :last-child { margin-bottom: 14px; }

/* Quiz */
.quiz { border: 1px solid var(--line); border-radius: 16px; background: var(--surface); padding: 6px 20px 18px; margin: 0 0 1.4em; }
.quiz-head { display: flex; justify-content: space-between; align-items: center; padding: 12px 0; border-bottom: 1px solid var(--line); margin-bottom: 8px; }
.quiz-head b { font-family: var(--font-head); }
.quiz-score { font-size: .82rem; color: var(--muted); }
.qz { padding: 14px 0; border-bottom: 1px dashed var(--line); }
.qz:last-child { border-bottom: 0; }
.qz p.q { font-weight: 600; margin: 0 0 10px; }
.qz .opts { display: grid; gap: 8px; }
.qz button.opt { text-align: left; padding: 10px 14px; border-radius: 10px; border: 1px solid var(--line); background: var(--bg); font-size: .9rem; }
.qz button.opt:hover:not(:disabled) { border-color: var(--accent); }
.qz button.opt:disabled { cursor: default; }
.qz button.opt.right { border-color: var(--good); background: color-mix(in srgb, var(--good) 14%, transparent); }
.qz button.opt.wrong { border-color: var(--bad); background: color-mix(in srgb, var(--bad) 12%, transparent); }
.qz .why { margin: 10px 0 0; font-size: .88rem; color: var(--ink-2); padding: 10px 12px; background: var(--surface-2); border-radius: 10px; }

/* Flashcards */
.cards-deck { display: grid; grid-template-columns: repeat(auto-fill, minmax(220px, 1fr)); gap: 12px; margin: 0 0 1.4em; }
.flash { perspective: 900px; min-height: 160px; border: 0; padding: 0; background: none; text-align: left; }
.flash-inner { position: relative; width: 100%; height: 100%; min-height: 160px; transition: transform .45s; transform-style: preserve-3d; }
.flash.flipped .flash-inner { transform: rotateY(180deg); }
.flash-face { position: absolute; inset: 0; backface-visibility: hidden; border-radius: 14px; border: 1px solid var(--line); padding: 16px; display: flex; flex-direction: column; justify-content: center; font-size: .9rem; line-height: 1.5; overflow: auto; }
.flash-front { background: var(--surface); font-family: var(--font-head); font-weight: 600; font-size: 1rem; }
.flash-front::after { content: 'tap to flip'; position: absolute; bottom: 10px; right: 14px; font-family: var(--font-body); font-weight: 400; font-size: .68rem; color: var(--muted); }
.flash-back { background: var(--accent-soft); transform: rotateY(180deg); color: var(--ink); }
@media (prefers-reduced-motion: reduce) { .flash-inner { transition: none; } }

/* Lesson footer */
.complete-box { display: flex; align-items: center; justify-content: space-between; gap: 16px; flex-wrap: wrap; margin: 40px 0 20px; padding: 18px 20px; border-radius: 14px; border: 1px solid var(--line); background: var(--surface); }
.complete-box p { margin: 0; font-size: .9rem; color: var(--ink-2); }
.btn.done { background: var(--accent-soft); color: var(--accent); border-color: var(--accent); }
.pager { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
.pager a { display: block; padding: 14px 18px; border: 1px solid var(--line); border-radius: 14px; background: var(--surface); color: var(--ink); text-decoration: none; }
.pager a:hover { border-color: var(--accent); }
.pager small { display: block; color: var(--muted); font-size: .74rem; }
.pager b { font-size: .9rem; }
.pager .next { text-align: right; grid-column: 2; }
.missing { padding: 40px; text-align: center; border: 1px dashed var(--line); border-radius: 16px; color: var(--ink-2); }
.skeleton { height: 14px; border-radius: 7px; background: var(--surface-2); margin: 12px 0; animation: pulse 1.2s ease-in-out infinite; }
@keyframes pulse { 50% { opacity: .5; } }

/* ---------- Responsive ---------- */
@media (max-width: 1180px) {
  .lesson-wrap { grid-template-columns: minmax(0, 760px); }
  .toc { display: none; }
}
@media (max-width: 900px) {
  .menu-toggle { display: inline-grid; }
  .layout { grid-template-columns: 1fr; }
  .sidebar { position: fixed; top: var(--top); left: 0; bottom: 0; height: auto; width: min(86vw, 320px); z-index: 45; transform: translateX(-100%); transition: transform .2s ease; box-shadow: var(--shadow); }
  body.nav-open .sidebar { transform: none; }
  body.nav-open .scrim { display: block; position: fixed; inset: var(--top) 0 0 0; background: rgba(0,0,0,.35); z-index: 44; }
  .view { padding: 24px 16px 40px; }
  .site-foot { padding: 20px 16px 32px; }
  .brand-text small { display: none; }
}
@media (max-width: 640px) {
  .topbar { gap: 6px; padding: 0 8px; }
  .brand-text { display: none; }
  .kbd-hint { display: none; }
  .search-wrap input { padding-right: 12px; }
  .top-actions #sizeDown, .top-actions #sizeUp { display: none; }
  .hero { padding: 30px 20px; }
  .ch-hero { padding: 22px 18px; }
  .pager { grid-template-columns: 1fr; }
  .pager .next { grid-column: 1; }
  .lesson-list .tag { display: none; }
}