/* ============================================================
   R & Seurat Academy — design system
   ============================================================ */
:root {
  --bg: #f6f7fb;
  --surface: #ffffff;
  --surface-2: #f1f3f9;
  --surface-3: #e8ebf4;
  --border: #e2e6f0;
  --border-strong: #cfd5e3;
  --text: #171c2c;
  --text-2: #4a5268;
  --text-3: #7d869c;
  --accent: #2563eb;
  --accent-soft: #e6eeff;
  --ok: #0e9f6e;
  --ok-soft: #e3f7ee;
  --warn: #c27803;
  --warn-soft: #fdf3dc;
  --bad: #d92d4b;
  --bad-soft: #fde8ec;
  --code-bg: #f4f6fb;
  --out-bg: #10162a;
  --out-text: #dbe3f7;
  --shadow-sm: 0 1px 2px rgba(20, 30, 60, .06);
  --shadow: 0 6px 24px -8px rgba(20, 30, 60, .14), 0 1px 3px rgba(20, 30, 60, .06);
  --radius: 14px;
  --radius-sm: 9px;
  --sidebar-w: 300px;
  --topbar-h: 58px;
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Inter, Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: "SF Mono", "JetBrains Mono", "Fira Code", Menlo, Consolas, monospace;
  /* syntax */
  --syn-kw: #7c3aed; --syn-str: #0a7f5a; --syn-num: #c2410c; --syn-com: #8a93a8;
  --syn-fn: #1d4ed8; --syn-op: #b0306a; --syn-var: #171c2c;
  --sec: var(--accent); --sec-soft: var(--accent-soft);
}
:root[data-theme="dark"] {
  --bg: #0c1120;
  --surface: #141a2e;
  --surface-2: #1a2139;
  --surface-3: #232b47;
  --border: #252e4a;
  --border-strong: #35405f;
  --text: #e8ecf8;
  --text-2: #aab3cb;
  --text-3: #7682a0;
  --accent: #6d9bff;
  --accent-soft: #1a2a52;
  --ok: #34d399; --ok-soft: #0f2f29;
  --warn: #fbbf24; --warn-soft: #33290e;
  --bad: #fb7185; --bad-soft: #3a1620;
  --code-bg: #10162a;
  --out-bg: #070b17;
  --out-text: #dbe3f7;
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, .3);
  --shadow: 0 8px 28px -8px rgba(0, 0, 0, .55);
  --syn-kw: #c4a5ff; --syn-str: #6ee7b7; --syn-num: #fdba74; --syn-com: #6b7694;
  --syn-fn: #8ab4ff; --syn-op: #f9a8d4; --syn-var: #e8ecf8;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: calc(var(--topbar-h) + 16px); }
body {
  margin: 0; background: var(--bg); color: var(--text);
  font-family: var(--font); font-size: 16px; line-height: 1.65;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }
button { font: inherit; color: inherit; cursor: pointer; }
h1, h2, h3, h4 { line-height: 1.25; letter-spacing: -.015em; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 6px; }

/* ---------- top bar ---------- */
.topbar {
  position: fixed; inset: 0 0 auto 0; height: var(--topbar-h); z-index: 50;
  display: flex; align-items: center; gap: 14px; padding: 0 18px;
  background: color-mix(in srgb, var(--surface) 88%, transparent);
  backdrop-filter: saturate(1.6) blur(12px);
  border-bottom: 1px solid var(--border);
}
.brand { display: flex; align-items: center; gap: 10px; color: var(--text); font-size: 16px; white-space: nowrap; }
.brand:hover { text-decoration: none; }
.brand-mark {
  width: 32px; height: 32px; border-radius: 9px; display: grid; place-items: center;
  background: linear-gradient(135deg, #2563eb, #7c3aed); color: #fff; font-weight: 800; font-size: 18px;
  box-shadow: 0 4px 12px -4px rgba(37, 99, 235, .6);
}
.brand-text b { font-weight: 700; }
.icon-btn {
  width: 36px; height: 36px; border-radius: 10px; border: 1px solid transparent; background: transparent;
  display: grid; place-items: center; color: var(--text-2);
}
.icon-btn:hover { background: var(--surface-2); color: var(--text); }
.menu-btn { display: none; }
.search {
  position: relative; flex: 1; max-width: 420px; margin-left: 18px;
  display: flex; align-items: center; gap: 8px; padding: 0 12px; height: 36px;
  background: var(--surface-2); border: 1px solid var(--border); border-radius: 10px; color: var(--text-3);
}
.search:focus-within { border-color: var(--accent); background: var(--surface); }
.search input { flex: 1; border: 0; background: transparent; outline: 0; color: var(--text); font: inherit; font-size: 14px; min-width: 0; }
.search-results {
  position: absolute; top: 42px; left: 0; right: 0; background: var(--surface); border: 1px solid var(--border);
  border-radius: 12px; box-shadow: var(--shadow); padding: 6px; max-height: 60vh; overflow: auto;
}
.search-results a { display: block; padding: 8px 10px; border-radius: 8px; color: var(--text); font-size: 14px; }
.search-results a small { display: block; color: var(--text-3); font-size: 12px; }
.search-results a:hover, .search-results a.active { background: var(--surface-2); text-decoration: none; }
.search-results .empty { padding: 10px; color: var(--text-3); font-size: 14px; }
.topbar-right { margin-left: auto; display: flex; align-items: center; gap: 8px; }
.r-status {
  display: flex; align-items: center; gap: 8px; height: 32px; padding: 0 12px; border-radius: 999px;
  border: 1px solid var(--border); background: var(--surface); font-size: 13px; color: var(--text-2); white-space: nowrap;
}
.r-status .dot { width: 8px; height: 8px; border-radius: 50%; background: var(--text-3); }
.r-status[data-state="loading"] .dot { background: var(--warn); animation: pulse 1s infinite; }
.r-status[data-state="ready"] .dot { background: var(--ok); }
.r-status[data-state="busy"] .dot { background: var(--accent); animation: pulse .7s infinite; }
.r-status[data-state="error"] .dot { background: var(--bad); }
@keyframes pulse { 50% { opacity: .3; transform: scale(.8); } }

/* ---------- layout ---------- */
.layout { display: flex; padding-top: var(--topbar-h); min-height: 100vh; }
.sidebar {
  position: fixed; top: var(--topbar-h); bottom: 0; left: 0; width: var(--sidebar-w);
  overflow-y: auto; padding: 16px 12px 40px; background: var(--surface); border-right: 1px solid var(--border);
}
.main { flex: 1; margin-left: var(--sidebar-w); min-width: 0; outline: 0; }
.boot { padding: 80px; text-align: center; color: var(--text-3); }
.scrim { display: none; }

.side-home {
  display: flex; align-items: center; gap: 10px; padding: 9px 12px; border-radius: 10px; color: var(--text-2); font-weight: 600; font-size: 14px;
}
.side-home:hover, .side-home.active { background: var(--surface-2); color: var(--text); text-decoration: none; }
.side-overall { margin: 10px 12px 14px; }
.side-overall .bar { height: 6px; }
.side-overall small { color: var(--text-3); font-size: 12px; display: flex; justify-content: space-between; margin-bottom: 5px; }
.side-sec { margin-top: 6px; }
.side-sec-head {
  width: 100%; display: flex; align-items: center; gap: 10px; padding: 9px 10px; border: 0; background: transparent;
  border-radius: 10px; text-align: left; font-weight: 650; font-size: 14.5px;
}
.side-sec-head:hover { background: var(--surface-2); }
.side-sec-head .sec-ico { width: 28px; height: 28px; border-radius: 8px; display: grid; place-items: center; background: var(--sec-soft); color: var(--sec); flex: none; }
.side-sec-head .sec-ico svg { width: 16px; height: 16px; }
.side-sec-head .t { flex: 1; }
.side-sec-head .count { font-size: 12px; color: var(--text-3); font-weight: 500; }
.side-sec-head .chev { color: var(--text-3); transition: transform .15s; }
.side-sec.open .chev { transform: rotate(90deg); }
.side-lessons { display: none; margin: 2px 0 8px 23px; padding-left: 12px; border-left: 2px solid var(--border); }
.side-sec.open .side-lessons { display: block; }
.side-lessons a {
  display: flex; align-items: center; gap: 9px; padding: 6px 10px; border-radius: 8px; color: var(--text-2); font-size: 13.5px; line-height: 1.35;
}
.side-lessons a:hover { background: var(--surface-2); color: var(--text); text-decoration: none; }
.side-lessons a.active { background: var(--sec-soft); color: var(--sec); font-weight: 600; }
.tick {
  width: 16px; height: 16px; flex: none; border-radius: 50%; border: 1.5px solid var(--border-strong); display: grid; place-items: center;
}
.tick.done { background: var(--ok); border-color: var(--ok); }
.tick.done::after { content: ""; width: 7px; height: 4px; border-left: 2px solid #fff; border-bottom: 2px solid #fff; transform: rotate(-45deg) translateY(-1px); }
.tick.part { border-color: var(--sec); background: var(--sec-soft); }

.bar { height: 8px; background: var(--surface-3); border-radius: 99px; overflow: hidden; }
.bar > i { display: block; height: 100%; background: linear-gradient(90deg, var(--sec), color-mix(in srgb, var(--sec) 60%, #a855f7)); border-radius: 99px; transition: width .4s; }

/* ---------- pages ---------- */
.page { max-width: 1120px; margin: 0 auto; padding: 36px 40px 100px; }
.hero {
  position: relative; overflow: hidden; border-radius: 22px; padding: 46px 48px; color: #fff;
  background: radial-gradient(1200px 400px at 90% -10%, rgba(255, 255, 255, .22), transparent 60%), linear-gradient(125deg, #1e40af 0%, #4f46e5 45%, #7c3aed 100%);
  box-shadow: 0 20px 50px -20px rgba(79, 70, 229, .55);
}
.hero h1 { font-size: clamp(30px, 4vw, 44px); margin: 0 0 12px; letter-spacing: -.03em; }
.hero p { font-size: 18px; max-width: 640px; margin: 0 0 26px; opacity: .93; }
.hero .cells { position: absolute; right: -10px; top: -10px; width: 380px; height: 300px; opacity: .9; pointer-events: none; }
.hero-actions { display: flex; flex-wrap: wrap; gap: 12px; align-items: center; }
.btn {
  display: inline-flex; align-items: center; gap: 8px; height: 40px; padding: 0 16px; border-radius: 10px;
  border: 1px solid var(--border-strong); background: var(--surface); color: var(--text); font-weight: 600; font-size: 14px;
  transition: transform .08s, box-shadow .15s, background .15s;
}
.btn:hover { text-decoration: none; background: var(--surface-2); }
.btn:active { transform: translateY(1px); }
.btn[disabled] { opacity: .55; cursor: default; }
.btn.primary { background: var(--accent); border-color: var(--accent); color: #fff; }
.btn.primary:hover { filter: brightness(1.08); background: var(--accent); }
.btn.ok { background: var(--ok); border-color: var(--ok); color: #fff; }
.btn.ghost { border-color: transparent; background: transparent; color: var(--text-2); }
.btn.ghost:hover { background: var(--surface-2); color: var(--text); }
.btn.sm { height: 32px; padding: 0 11px; font-size: 13px; border-radius: 8px; }
.btn.white { background: #fff; color: #1e3a8a; border-color: #fff; height: 46px; padding: 0 22px; font-size: 15px; }
.btn.glass { background: rgba(255, 255, 255, .14); border-color: rgba(255, 255, 255, .3); color: #fff; height: 46px; padding: 0 20px; }
.btn.glass:hover { background: rgba(255, 255, 255, .22); }
.hero-stats { display: flex; gap: 28px; margin-top: 30px; flex-wrap: wrap; }
.hero-stats div { font-size: 13px; opacity: .9; }
.hero-stats b { display: block; font-size: 24px; letter-spacing: -.02em; }

.h-row { display: flex; align-items: baseline; justify-content: space-between; margin: 44px 0 16px; }
.h-row h2 { margin: 0; font-size: 22px; }
.h-row span { color: var(--text-3); font-size: 14px; }

.sec-grid { display: grid; grid-template-columns: repeat(2, 1fr); gap: 18px; }
.sec-card {
  position: relative; display: flex; flex-direction: column; gap: 10px; padding: 24px; border-radius: var(--radius);
  background: var(--surface); border: 1px solid var(--border); color: var(--text); box-shadow: var(--shadow-sm);
  transition: transform .15s, box-shadow .15s, border-color .15s; overflow: hidden;
}
.sec-card::before { content: ""; position: absolute; inset: 0 0 auto 0; height: 4px; background: var(--sec); }
.sec-card:hover { transform: translateY(-3px); box-shadow: var(--shadow); border-color: var(--sec); text-decoration: none; }
.sec-card .top { display: flex; align-items: center; gap: 14px; }
.sec-card .sec-ico { width: 48px; height: 48px; border-radius: 13px; display: grid; place-items: center; background: var(--sec-soft); color: var(--sec); flex: none; }
.sec-card .sec-ico svg { width: 26px; height: 26px; }
.sec-card .num { font-size: 12px; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; color: var(--sec); }
.sec-card h3 { margin: 0; font-size: 21px; }
.sec-card p { margin: 0; color: var(--text-2); font-size: 15px; flex: 1; }
.sec-card .meta { display: flex; justify-content: space-between; font-size: 13px; color: var(--text-3); margin-top: 6px; }

.how { display: grid; grid-template-columns: repeat(4, 1fr); gap: 14px; }
.how div { padding: 18px; border-radius: var(--radius); background: var(--surface); border: 1px solid var(--border); }
.how b { display: block; margin-bottom: 4px; }
.how .n { width: 28px; height: 28px; border-radius: 8px; background: var(--accent-soft); color: var(--accent); display: grid; place-items: center; font-weight: 700; font-size: 14px; margin-bottom: 10px; }
.how p { margin: 0; color: var(--text-2); font-size: 14px; }

/* section page */
.sec-hero { display: flex; gap: 20px; align-items: center; padding: 30px; border-radius: 20px; background: linear-gradient(135deg, var(--sec-soft), var(--surface)); border: 1px solid var(--border); }
.sec-hero .sec-ico { width: 68px; height: 68px; border-radius: 18px; display: grid; place-items: center; background: var(--sec); color: #fff; flex: none; }
.sec-hero .sec-ico svg { width: 36px; height: 36px; }
.sec-hero h1 { margin: 0 0 4px; font-size: 32px; }
.sec-hero p { margin: 0; color: var(--text-2); max-width: 680px; }
.sec-hero .bar { margin-top: 14px; max-width: 360px; }
.lesson-list { margin-top: 26px; display: grid; gap: 10px; }
.lesson-row {
  display: flex; align-items: center; gap: 16px; padding: 16px 20px; border-radius: var(--radius); background: var(--surface);
  border: 1px solid var(--border); color: var(--text); transition: border-color .15s, transform .12s;
}
.lesson-row:hover { border-color: var(--sec); text-decoration: none; transform: translateX(3px); }
.lesson-row .idx { width: 36px; height: 36px; border-radius: 10px; display: grid; place-items: center; background: var(--sec-soft); color: var(--sec); font-weight: 700; flex: none; font-size: 14px; }
.lesson-row.done .idx { background: var(--ok); color: #fff; }
.lesson-row .body { flex: 1; min-width: 0; }
.lesson-row .body b { display: block; font-size: 16px; }
.lesson-row .body span { color: var(--text-2); font-size: 14px; }
.lesson-row .tags { display: flex; gap: 8px; flex: none; }
.chip { display: inline-flex; align-items: center; height: 24px; padding: 0 10px; border-radius: 99px; background: var(--surface-2); color: var(--text-2); font-size: 12px; font-weight: 600; white-space: nowrap; }
.chip.lvl-beginner { background: var(--ok-soft); color: var(--ok); }
.chip.lvl-intermediate { background: var(--warn-soft); color: var(--warn); }
.chip.lvl-advanced { background: var(--bad-soft); color: var(--bad); }
.chip.live { background: var(--accent-soft); color: var(--accent); }

/* ---------- lesson ---------- */
.lesson-wrap { display: grid; grid-template-columns: minmax(0, 780px) 220px; gap: 48px; justify-content: center; max-width: 1140px; margin: 0 auto; padding: 30px 40px 120px; }
.crumbs { font-size: 13px; color: var(--text-3); display: flex; gap: 8px; align-items: center; flex-wrap: wrap; }
.crumbs a { color: var(--sec); font-weight: 600; }
.lesson-head h1 { font-size: clamp(28px, 3.4vw, 38px); margin: 10px 0 12px; letter-spacing: -.028em; }
.lesson-head .lead { font-size: 18px; color: var(--text-2); margin: 0 0 16px; }
.lesson-meta { display: flex; gap: 8px; flex-wrap: wrap; padding-bottom: 22px; border-bottom: 1px solid var(--border); margin-bottom: 8px; }
.toc { position: sticky; top: calc(var(--topbar-h) + 30px); align-self: start; font-size: 13.5px; max-height: calc(100vh - 120px); overflow: auto; }
.toc h4 { margin: 0 0 10px; font-size: 11.5px; letter-spacing: .1em; text-transform: uppercase; color: var(--text-3); }
.toc a { display: block; padding: 5px 0 5px 12px; color: var(--text-2); border-left: 2px solid var(--border); line-height: 1.35; }
.toc a:hover { color: var(--text); text-decoration: none; }
.toc a.active { color: var(--sec); border-left-color: var(--sec); font-weight: 600; }
.toc .prog { margin-top: 18px; padding: 14px; border-radius: 12px; background: var(--surface); border: 1px solid var(--border); }
.toc .prog small { color: var(--text-3); display: block; margin-bottom: 6px; }

.prose { font-size: 16.5px; }
.prose h2 { font-size: 25px; margin: 46px 0 12px; padding-top: 6px; }
.prose h3 { font-size: 19px; margin: 30px 0 8px; }
.prose p { margin: 12px 0; }
.prose ul, .prose ol { padding-left: 24px; margin: 12px 0; }
.prose li { margin: 5px 0; }
.prose li::marker { color: var(--sec); font-weight: 600; }
.prose strong { font-weight: 650; }
.prose img { max-width: 100%; border-radius: 10px; }
.prose hr { border: 0; border-top: 1px solid var(--border); margin: 36px 0; }
.prose :not(pre) > code {
  font-family: var(--mono); font-size: .86em; padding: 2px 6px; border-radius: 6px; background: var(--surface-3); color: var(--syn-op);
}
.prose table { width: 100%; border-collapse: collapse; margin: 18px 0; font-size: 14.5px; display: block; overflow-x: auto; }
.prose th { text-align: left; background: var(--surface-2); font-weight: 650; }
.prose th, .prose td { padding: 9px 12px; border: 1px solid var(--border); vertical-align: top; }
.prose blockquote { margin: 16px 0; padding: 4px 18px; border-left: 3px solid var(--border-strong); color: var(--text-2); }

/* callouts */
.callout { margin: 20px 0; padding: 14px 18px 14px 50px; border-radius: 12px; position: relative; background: var(--accent-soft); border: 1px solid color-mix(in srgb, var(--accent) 25%, transparent); font-size: 15.5px; }
.callout::before { content: "i"; position: absolute; left: 16px; top: 15px; width: 22px; height: 22px; border-radius: 50%; display: grid; place-items: center; background: var(--accent); color: #fff; font-weight: 700; font-size: 13px; font-family: Georgia, serif; }
.callout > :first-child { margin-top: 0; } .callout > :last-child { margin-bottom: 0; }
.callout .c-title { display: block; font-weight: 700; margin-bottom: 2px; }
.callout.tip { background: var(--ok-soft); border-color: color-mix(in srgb, var(--ok) 30%, transparent); }
.callout.tip::before { content: "✓"; background: var(--ok); font-family: var(--font); }
.callout.warning { background: var(--warn-soft); border-color: color-mix(in srgb, var(--warn) 35%, transparent); }
.callout.warning::before { content: "!"; background: var(--warn); font-family: var(--font); }
.callout.key { background: var(--surface); border: 1px solid var(--border); border-left: 4px solid var(--sec); padding-left: 22px; box-shadow: var(--shadow-sm); }
.callout.key::before { display: none; }
.callout.bio { background: color-mix(in srgb, #14b8a6 10%, var(--surface)); border-color: color-mix(in srgb, #14b8a6 35%, transparent); }
.callout.bio::before { content: "🧬"; background: transparent; font-size: 18px; }
.callout.source { background: var(--surface-2); border-color: var(--border); }
.callout.source::before { content: "↗"; background: var(--text-3); font-family: var(--font); }

/* code */
.code {
  position: relative; margin: 16px 0; border-radius: 12px; background: var(--code-bg); border: 1px solid var(--border); overflow: hidden;
}
.code pre { margin: 0; padding: 14px 16px; overflow-x: auto; font-family: var(--mono); font-size: 13.8px; line-height: 1.6; }
.code .copy { position: absolute; top: 8px; right: 8px; opacity: 0; transition: opacity .15s; }
.code:hover .copy, .code .copy:focus-visible { opacity: 1; }
.code.out { background: var(--out-bg); border-color: transparent; }
.code.out pre { color: var(--out-text); }
.code-label { display: flex; align-items: center; gap: 8px; padding: 7px 14px; font-size: 12px; font-weight: 600; color: var(--text-3); border-bottom: 1px solid var(--border); background: var(--surface-2); letter-spacing: .03em; }
.code-label .where { margin-left: auto; font-weight: 500; }

.cm-keyword { color: var(--syn-kw) !important; font-weight: 600; }
.cm-string { color: var(--syn-str) !important; }
.cm-number, .cm-atom { color: var(--syn-num) !important; }
.cm-comment { color: var(--syn-com) !important; font-style: italic; }
.cm-builtin { color: var(--syn-fn) !important; }
.cm-operator, .cm-arrow, .cm-dollar { color: var(--syn-op) !important; }
.cm-variable, .cm-variable-2, .cm-variable-3 { color: var(--syn-var) !important; }
.cm-error { color: inherit !important; }

/* runnable cells */
.cell { margin: 18px 0; border-radius: 12px; border: 1px solid var(--border-strong); background: var(--surface); overflow: hidden; box-shadow: var(--shadow-sm); }
.cell:focus-within { border-color: var(--sec); }
.cell-bar { display: flex; align-items: center; gap: 8px; padding: 6px 8px 6px 14px; background: var(--surface-2); border-bottom: 1px solid var(--border); }
.cell-bar .lbl { font-size: 12px; font-weight: 700; color: var(--text-3); letter-spacing: .06em; text-transform: uppercase; display: flex; gap: 7px; align-items: center; }
.cell-bar .lbl i { width: 7px; height: 7px; border-radius: 50%; background: var(--ok); display: inline-block; }
.cell-bar .sp { flex: 1; }
.cell-bar kbd { font: 11px var(--mono); color: var(--text-3); padding: 2px 6px; border: 1px solid var(--border); border-radius: 5px; }
.CodeMirror { height: auto; font-family: var(--mono); font-size: 13.8px; line-height: 1.6; background: var(--surface); color: var(--text); }
.CodeMirror-scroll { min-height: 44px; }
.CodeMirror-lines { padding: 10px 0; }
.CodeMirror pre.CodeMirror-line, .CodeMirror pre.CodeMirror-line-like { padding: 0 14px; }
.CodeMirror-gutters { background: var(--surface); border-right: 0; }
.CodeMirror-linenumber { color: var(--text-3); opacity: .6; }
.CodeMirror-cursor { border-left-color: var(--text); }
.CodeMirror-selected, .CodeMirror-focused .CodeMirror-selected { background: color-mix(in srgb, var(--accent) 22%, transparent) !important; }
.output { border-top: 1px solid var(--border); background: var(--out-bg); color: var(--out-text); padding: 12px 16px; font-family: var(--mono); font-size: 13.2px; line-height: 1.55; max-height: 520px; overflow: auto; }
.output:empty { display: none; }
.output pre { margin: 0; white-space: pre-wrap; word-break: break-word; font: inherit; }
.output .err { color: #ff8fa3; }
.output .msg { color: #fcd58a; }
.output .muted { color: #8090b4; font-style: italic; }
.output canvas { display: block; max-width: 100%; width: 560px; height: auto; background: #fff; border-radius: 8px; margin: 8px 0; }
.spinner { display: inline-block; width: 13px; height: 13px; border: 2px solid currentColor; border-right-color: transparent; border-radius: 50%; animation: spin .7s linear infinite; vertical-align: -2px; margin-right: 8px; }
@keyframes spin { to { transform: rotate(360deg); } }

/* exercises */
.exercise { margin: 28px 0; border-radius: 16px; border: 1px solid var(--border-strong); background: var(--surface); box-shadow: var(--shadow); overflow: hidden; }
.exercise.passed { border-color: var(--ok); }
.ex-head { display: flex; align-items: center; gap: 12px; padding: 14px 18px; background: linear-gradient(90deg, var(--sec-soft), transparent); border-bottom: 1px solid var(--border); }
.ex-badge { font-size: 11.5px; font-weight: 800; letter-spacing: .08em; text-transform: uppercase; color: #fff; background: var(--sec); padding: 4px 10px; border-radius: 7px; white-space: nowrap; }
.ex-head h4 { margin: 0; font-size: 16.5px; flex: 1; }
.ex-state { font-size: 13px; font-weight: 700; color: var(--ok); display: none; align-items: center; gap: 6px; }
.exercise.passed .ex-state { display: flex; }
.ex-prompt { padding: 14px 18px 4px; font-size: 15.5px; }
.ex-prompt p { margin: 8px 0; }
.ex-editor { border-top: 1px solid var(--border); border-bottom: 1px solid var(--border); margin-top: 10px; }
.ex-actions { display: flex; flex-wrap: wrap; gap: 8px; padding: 12px 14px; align-items: center; }
.ex-actions .sp { flex: 1; }
.ex-feedback { margin: 0 14px 14px; padding: 12px 16px; border-radius: 10px; font-size: 14.5px; display: none; }
.ex-feedback.show { display: block; }
.ex-feedback.good { background: var(--ok-soft); color: var(--ok); font-weight: 600; }
.ex-feedback.bad { background: var(--bad-soft); color: var(--bad); }
.ex-feedback.hint { background: var(--warn-soft); color: var(--text); }
.ex-feedback.hint b { color: var(--warn); }
.ex-feedback p { margin: 4px 0; }
.exercise .output { border-top: 1px solid var(--border); }
.ex-local { padding: 0 18px 4px; font-size: 13.5px; color: var(--text-3); }

.ex-solution { margin: 0 14px 14px; }
.ex-solution .code { margin: 0 0 10px; }
.ex-expected { padding: 12px 16px; border-radius: 10px; background: var(--ok-soft); font-size: 14.5px; }
.ex-expected p { margin: 4px 0; display: inline; }

/* quiz */
.quiz { margin: 28px 0; padding: 20px 22px; border-radius: 16px; background: var(--surface); border: 1px solid var(--border-strong); box-shadow: var(--shadow-sm); }
.quiz .q-badge { font-size: 11.5px; font-weight: 800; letter-spacing: .08em; text-transform: uppercase; color: var(--sec); }
.quiz .q-text { font-weight: 650; font-size: 16.5px; margin: 6px 0 14px; }
.quiz .q-text p { margin: 0; }
.quiz .opts { display: grid; gap: 8px; }
.quiz .opt { text-align: left; padding: 11px 14px; border-radius: 10px; border: 1.5px solid var(--border); background: var(--surface); font-size: 15px; display: flex; gap: 12px; align-items: baseline; transition: border-color .12s, background .12s; }
.quiz .opt:hover:not([disabled]) { border-color: var(--sec); background: var(--sec-soft); }
.quiz .opt .k { font-weight: 700; color: var(--text-3); font-size: 13px; }
.quiz .opt p { margin: 0; display: inline; }
.quiz .opt.right { border-color: var(--ok); background: var(--ok-soft); }
.quiz .opt.wrong { border-color: var(--bad); background: var(--bad-soft); }
.quiz .opt[disabled] { cursor: default; }
.quiz .q-exp { margin-top: 12px; padding: 12px 14px; border-radius: 10px; background: var(--surface-2); font-size: 14.5px; display: none; }
.quiz.answered .q-exp { display: block; }
.quiz .q-exp p { margin: 0; }

/* lesson footer */
.lesson-foot { margin-top: 56px; padding-top: 28px; border-top: 1px solid var(--border); }
.complete-box { display: flex; align-items: center; gap: 16px; padding: 20px 22px; border-radius: 16px; background: var(--surface); border: 1px solid var(--border); flex-wrap: wrap; }
.complete-box .txt { flex: 1; min-width: 220px; }
.complete-box b { display: block; font-size: 17px; }
.complete-box span { color: var(--text-2); font-size: 14px; }
.pn { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin-top: 18px; }
.pn a { padding: 16px 18px; border-radius: 14px; border: 1px solid var(--border); background: var(--surface); color: var(--text); }
.pn a:hover { border-color: var(--sec); text-decoration: none; }
.pn a small { display: block; color: var(--text-3); font-size: 12px; text-transform: uppercase; letter-spacing: .08em; font-weight: 700; }
.pn a.next { text-align: right; }
.pn .ghost-cell { visibility: hidden; }

.local-banner { margin: 18px 0; padding: 14px 18px; border-radius: 12px; background: var(--warn-soft); border: 1px solid color-mix(in srgb, var(--warn) 35%, transparent); font-size: 15px; }
.toast { position: fixed; bottom: 24px; left: 50%; transform: translateX(-50%) translateY(20px); background: var(--text); color: var(--bg); padding: 10px 18px; border-radius: 10px; font-size: 14px; font-weight: 600; opacity: 0; transition: all .25s; z-index: 100; pointer-events: none; }
.toast.show { opacity: 1; transform: translateX(-50%); }

/* ---------- responsive ---------- */
@media (max-width: 1180px) { .lesson-wrap { grid-template-columns: minmax(0, 780px); } .toc { display: none; } }
@media (max-width: 900px) {
  .menu-btn { display: grid; }
  .sidebar { transform: translateX(-102%); transition: transform .22s; z-index: 60; box-shadow: var(--shadow); }
  body.nav-open .sidebar { transform: none; }
  body.nav-open .scrim { display: block; position: fixed; inset: var(--topbar-h) 0 0 0; background: rgba(0, 0, 0, .4); z-index: 55; }
  .main { margin-left: 0; }
  .page, .lesson-wrap { padding-left: 16px; padding-right: 16px; }
  .sec-grid { grid-template-columns: 1fr; }
  .how { grid-template-columns: 1fr 1fr; }
  .hero { padding: 30px 24px; }
  .hero .cells { display: none; }
  .brand-text { display: none; }
  .r-status .label { display: none; }
  .search { margin-left: 0; }
  .lesson-row .tags { display: none; }
  .pn { grid-template-columns: 1fr; }
}
@media (prefers-reduced-motion: reduce) { * { animation: none !important; transition: none !important; scroll-behavior: auto !important; } }
@media print { .topbar, .sidebar, .toc, .ex-actions, .lesson-foot { display: none !important; } .main { margin: 0; } .layout { padding: 0; } }