/* ── Tokens ─────────────────────────────────────────── */
:root {
  --bg: #f6f4ef;
  --bg-elev: #ffffff;
  --bg-muted: #ebe7de;
  --ink: #1a1c1e;
  --ink-soft: #4a5058;
  --ink-faint: #7a828c;
  --line: #ddd6c8;
  --accent: #0b6e4f;
  --accent-soft: #d8f3e7;
  --accent-2: #c45c26;
  --accent-2-soft: #fde8d8;
  --secant: #c45c26;
  --tangent: #1d4ed8;
  --curve: #0b6e4f;
  --danger: #b42318;
  --shadow: 0 12px 40px rgba(26, 28, 30, 0.08);
  --radius: 14px;
  --radius-sm: 10px;
  --font: "DM Sans", system-ui, sans-serif;
  --serif: "Literata", Georgia, serif;
  --mono: "JetBrains Mono", ui-monospace, monospace;
  --max: 1100px;
  --header-h: 64px;
}

[data-theme="dark"] {
  --bg: #121416;
  --bg-elev: #1c1f24;
  --bg-muted: #262a31;
  --ink: #ece8e1;
  --ink-soft: #b0b6bf;
  --ink-faint: #7a828c;
  --line: #333942;
  --accent: #3ecf8e;
  --accent-soft: #163528;
  --accent-2: #f0a06a;
  --accent-2-soft: #3a281c;
  --secant: #f0a06a;
  --tangent: #7eb6ff;
  --curve: #3ecf8e;
  --danger: #f97066;
  --shadow: 0 12px 40px rgba(0, 0, 0, 0.35);
}

/* ── Reset / base ───────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: var(--font);
  background: var(--bg);
  color: var(--ink);
  line-height: 1.6;
  min-height: 100vh;
}
img, canvas { max-width: 100%; display: block; }
a { color: var(--accent); text-decoration-thickness: 1px; text-underline-offset: 3px; }
a:hover { color: var(--accent-2); }
button, input, select { font: inherit; color: inherit; }
.skip-link {
  position: absolute; left: -999px; top: 0;
  background: var(--accent); color: #fff; padding: 0.5rem 1rem; z-index: 100;
}
.skip-link:focus { left: 0.5rem; top: 0.5rem; }

/* ── Header ─────────────────────────────────────────── */
.site-header {
  position: sticky; top: 0; z-index: 50;
  background: color-mix(in srgb, var(--bg) 88%, transparent);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--line);
}
.header-inner {
  max-width: calc(var(--max) + 4rem);
  margin: 0 auto;
  padding: 0 1.25rem;
  height: var(--header-h);
  display: flex; align-items: center; gap: 1rem;
}
.logo {
  display: flex; align-items: center; gap: 0.6rem;
  text-decoration: none; color: var(--ink); font-weight: 600;
  flex-shrink: 0;
}
.logo-mark {
  font-family: var(--mono);
  font-size: 0.85rem;
  background: var(--accent);
  color: #fff;
  padding: 0.3rem 0.5rem;
  border-radius: 8px;
  letter-spacing: -0.02em;
}
[data-theme="dark"] .logo-mark { color: #0a0c0e; }
.logo-text { font-size: 0.95rem; }
.main-nav {
  display: flex; gap: 0.15rem; margin-left: auto; flex-wrap: wrap;
}
.main-nav a {
  text-decoration: none; color: var(--ink-soft);
  padding: 0.4rem 0.7rem; border-radius: 8px; font-size: 0.9rem; font-weight: 500;
}
.main-nav a:hover, .main-nav a.active {
  color: var(--ink); background: var(--bg-muted);
}
.main-nav a.home-link {
  color: var(--ink-soft);
  font-weight: 600;
  padding-right: 1rem;
  margin-right: 0.4rem;
  border-right: 1px solid var(--line);
}
.main-nav a.home-link:hover { color: var(--ink); background: none; }
.theme-toggle, .nav-toggle {
  background: var(--bg-muted); border: 1px solid var(--line);
  border-radius: 10px; width: 40px; height: 40px;
  display: grid; place-items: center; cursor: pointer;
}
.nav-toggle { display: none; flex-direction: column; gap: 5px; padding: 10px; }
.nav-toggle span {
  display: block; width: 18px; height: 2px; background: var(--ink); border-radius: 1px;
}

/* ── Layout panels ──────────────────────────────────── */
main { max-width: calc(var(--max) + 4rem); margin: 0 auto; padding: 1.5rem 1.25rem 4rem; }
.panel { display: none; animation: fadeIn 0.35s ease; }
.panel.active { display: block; }
@keyframes fadeIn {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: none; }
}

.section-header { margin-bottom: 2rem; max-width: 42rem; }
.eyebrow {
  font-size: 0.78rem; font-weight: 600; letter-spacing: 0.08em;
  text-transform: uppercase; color: var(--accent); margin: 0 0 0.5rem;
}
h1 {
  font-family: var(--serif); font-weight: 600; font-size: clamp(1.8rem, 4vw, 2.6rem);
  line-height: 1.2; margin: 0 0 0.75rem; letter-spacing: -0.02em;
}
.lede { font-size: 1.1rem; color: var(--ink-soft); margin: 0; }
.hero { max-width: 40rem; margin-bottom: 2.5rem; }
.hero-actions { display: flex; flex-wrap: wrap; gap: 0.75rem; margin-top: 1.5rem; }

/* ── Buttons ────────────────────────────────────────── */
.btn {
  display: inline-flex; align-items: center; justify-content: center;
  padding: 0.7rem 1.15rem; border-radius: 999px; font-weight: 600;
  font-size: 0.95rem; text-decoration: none; border: 1px solid transparent;
  cursor: pointer; transition: transform 0.15s, background 0.15s;
}
.btn:hover { transform: translateY(-1px); }
.btn.primary { background: var(--accent); color: #fff; }
[data-theme="dark"] .btn.primary { color: #0a0c0e; }
.btn.ghost { background: transparent; border-color: var(--line); color: var(--ink); }
.btn.ghost:hover { background: var(--bg-muted); }
.btn.small { padding: 0.45rem 0.9rem; font-size: 0.85rem; }

/* ── Cards ──────────────────────────────────────────── */
.card-grid {
  display: grid; grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 1rem; margin-bottom: 2rem;
}
.card, .mini-card, .callout, .example-box, .formula-block {
  background: var(--bg-elev); border: 1px solid var(--line);
  border-radius: var(--radius); box-shadow: var(--shadow);
}
.card { padding: 1.25rem 1.35rem; }
.link-card { cursor: pointer; transition: border-color 0.15s, transform 0.15s; }
.link-card:hover { border-color: var(--accent); transform: translateY(-2px); }
.card-icon {
  width: 36px; height: 36px; border-radius: 10px;
  background: var(--accent-soft); color: var(--accent);
  display: grid; place-items: center; font-weight: 700; margin-bottom: 0.75rem;
}
.card h2 { font-size: 1.1rem; margin: 0 0 0.4rem; font-family: var(--serif); }
.card p { margin: 0; color: var(--ink-soft); font-size: 0.95rem; }
.callout {
  padding: 1.1rem 1.25rem; background: var(--accent-soft);
  border-color: color-mix(in srgb, var(--accent) 30%, var(--line));
  color: var(--ink); font-size: 0.95rem;
}
.callout strong { color: var(--accent); }

/* ── Prose ──────────────────────────────────────────── */
.prose { max-width: 42rem; }
.prose h2 {
  font-family: var(--serif); font-size: 1.45rem; margin: 2.2rem 0 0.75rem;
  letter-spacing: -0.01em;
}
.prose h3 { font-size: 1.05rem; margin: 0 0 0.5rem; }
.prose p, .prose li { color: var(--ink-soft); }
.prose p strong, .prose li strong { color: var(--ink); }
.prose ul, .prose ol { padding-left: 1.2rem; }
.prose li { margin-bottom: 0.4rem; }
.law-list li { margin-bottom: 0.55rem; }
.two-col {
  display: grid; grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 1rem; margin: 1.25rem 0;
}
.mini-card { padding: 1.1rem 1.2rem; }
.mini-card p { margin: 0; font-size: 0.92rem; color: var(--ink-soft); }
.example-box {
  padding: 1.2rem 1.35rem; margin: 1.25rem 0;
  border-left: 4px solid var(--accent);
}
.example-box h3 { margin: 0 0 0.6rem; font-size: 1rem; }
.example-box p { margin: 0.75rem 0 0; font-size: 0.95rem; }
.formula-block {
  padding: 1.1rem 1.25rem; margin: 1.25rem 0; text-align: center;
  overflow-x: auto;
}
.formula-block.highlight {
  background: linear-gradient(135deg, var(--accent-soft), var(--bg-elev));
  border-color: color-mix(in srgb, var(--accent) 35%, var(--line));
}
.formula-label {
  font-size: 0.75rem; font-weight: 600; letter-spacing: 0.06em;
  text-transform: uppercase; color: var(--accent); margin: 0 0 0.5rem;
}
.formula-caption {
  font-size: 0.9rem; color: var(--ink-soft); margin: 0.75rem 0 0; text-align: left;
}
.formula.small { font-size: 0.95rem; margin: 0.5rem 0; overflow-x: auto; }
.steps { padding-left: 1.25rem; }
.steps li { margin-bottom: 0.85rem; }
.nav-row { display: flex; flex-wrap: wrap; gap: 0.75rem; margin-top: 2.5rem; }

/* ── Geometry demo ──────────────────────────────────── */
.demo-layout {
  display: grid; grid-template-columns: 1.4fr 1fr; gap: 1.25rem;
  margin-bottom: 2.5rem; align-items: start;
}
.demo-stage {
  background: var(--bg-elev); border: 1px solid var(--line);
  border-radius: var(--radius); box-shadow: var(--shadow); overflow: hidden;
}
#derivCanvas {
  width: 100%; height: auto; background:
    radial-gradient(circle at 20% 20%, color-mix(in srgb, var(--accent) 8%, transparent), transparent 50%),
    var(--bg-elev);
  cursor: crosshair;
}
.demo-controls {
  display: grid; gap: 0.85rem; padding: 1rem 1.15rem 1.2rem;
  border-top: 1px solid var(--line); background: var(--bg-muted);
}
.control { display: grid; gap: 0.35rem; font-size: 0.9rem; }
.control span { color: var(--ink-soft); }
.control strong { color: var(--ink); font-family: var(--mono); font-size: 0.88rem; }
.control input[type="range"] { width: 100%; accent-color: var(--accent); }
.control select {
  background: var(--bg-elev); border: 1px solid var(--line);
  border-radius: 8px; padding: 0.45rem 0.6rem;
}
.demo-readout {
  background: var(--bg-elev); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 1.2rem 1.3rem; box-shadow: var(--shadow);
  position: sticky; top: calc(var(--header-h) + 1rem);
}
.demo-readout h2 { font-size: 1rem; margin: 0 0 1rem; font-family: var(--serif); }
.readout { margin: 0; display: grid; gap: 0.65rem; }
.readout > div {
  display: flex; justify-content: space-between; gap: 1rem;
  font-size: 0.9rem; border-bottom: 1px dashed var(--line); padding-bottom: 0.5rem;
}
.readout dt { color: var(--ink-faint); }
.readout dd {
  margin: 0; font-family: var(--mono); font-size: 0.82rem;
  text-align: right; color: var(--ink);
}
.hint {
  margin: 1rem 0 0; padding: 0.75rem;
  background: var(--accent-2-soft); border-radius: var(--radius-sm);
  font-size: 0.88rem; color: var(--ink-soft);
}
.legend {
  display: grid; gap: 0.4rem; margin-top: 1.1rem; font-size: 0.82rem; color: var(--ink-soft);
}
.legend span { display: flex; align-items: center; gap: 0.5rem; }
.swatch {
  width: 14px; height: 14px; border-radius: 4px; display: inline-block;
}
.swatch.curve { background: var(--curve); }
.swatch.secant { background: var(--secant); }
.swatch.tangent { background: var(--tangent); }
.swatch.point { background: var(--ink); border-radius: 50%; }

/* ── Catalog ────────────────────────────────────────── */
.catalog-toolbar { margin-bottom: 1.5rem; }
#fnSearch {
  width: 100%; max-width: 420px;
  padding: 0.7rem 1rem; border-radius: 999px;
  border: 1px solid var(--line); background: var(--bg-elev);
  margin-bottom: 0.9rem;
}
#fnSearch:focus { outline: 2px solid var(--accent); outline-offset: 2px; }
.chip-row { display: flex; flex-wrap: wrap; gap: 0.45rem; margin-bottom: 0.75rem; }
.chip {
  border: 1px solid var(--line); background: var(--bg-elev);
  color: var(--ink-soft); border-radius: 999px;
  padding: 0.35rem 0.8rem; font-size: 0.82rem; font-weight: 500;
  cursor: pointer;
}
.chip:hover { border-color: var(--accent); color: var(--ink); }
.chip.active {
  background: var(--accent); color: #fff; border-color: var(--accent);
}
[data-theme="dark"] .chip.active { color: #0a0c0e; }
.catalog-meta { font-size: 0.85rem; color: var(--ink-faint); margin: 0; }

.fn-list { display: grid; gap: 0.85rem; }
.fn-card {
  background: var(--bg-elev); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 1.15rem 1.25rem;
  box-shadow: var(--shadow);
}
.fn-card-header {
  display: flex; flex-wrap: wrap; align-items: baseline; gap: 0.5rem 0.85rem;
  margin-bottom: 0.65rem;
}
.fn-id {
  font-family: var(--mono); font-size: 0.75rem; color: var(--ink-faint);
}
.fn-name {
  font-family: var(--serif); font-size: 1.15rem; font-weight: 600; margin: 0;
}
.fn-cat {
  font-size: 0.72rem; font-weight: 600; letter-spacing: 0.04em;
  text-transform: uppercase; color: var(--accent);
  background: var(--accent-soft); padding: 0.2rem 0.55rem; border-radius: 999px;
}
.fn-grid {
  display: grid; grid-template-columns: 1fr; gap: 0.65rem;
}
@media (min-width: 700px) {
  .fn-grid { grid-template-columns: 1fr 1fr; }
}
.fn-block {
  background: var(--bg-muted); border-radius: var(--radius-sm);
  padding: 0.75rem 0.9rem; overflow-x: auto;
}
.fn-block .label {
  font-size: 0.72rem; font-weight: 600; letter-spacing: 0.05em;
  text-transform: uppercase; color: var(--ink-faint); margin: 0 0 0.35rem;
}
.fn-intuit {
  grid-column: 1 / -1;
  font-size: 0.92rem; color: var(--ink-soft);
  margin: 0.15rem 0 0; padding-top: 0.35rem;
  border-top: 1px dashed var(--line);
}
.fn-intuit strong { color: var(--ink); }

/* ── Walkthrough UI ─────────────────────────────────── */
.fn-card-top {
  display: flex; flex-wrap: wrap; align-items: flex-start;
  justify-content: space-between; gap: 0.75rem 1rem;
  margin-bottom: 0.75rem;
}
.fn-card-top .fn-card-header { margin-bottom: 0; flex: 1; min-width: 200px; }
.fn-card.is-open {
  border-color: color-mix(in srgb, var(--accent) 40%, var(--line));
  box-shadow: var(--shadow), 0 0 0 1px color-mix(in srgb, var(--accent) 15%, transparent);
}
.fn-walkthrough {
  margin-top: 1.1rem;
  padding-top: 1.1rem;
  border-top: 1px solid var(--line);
}
.walk-layout {
  display: grid;
  grid-template-columns: 1.4fr 0.9fr;
  gap: 1.1rem;
  align-items: start;
}
.walk-tabs {
  display: flex; flex-wrap: wrap; gap: 0.4rem;
  margin-bottom: 0.85rem;
}
.walk-tab {
  border: 1px solid var(--line);
  background: var(--bg-muted);
  color: var(--ink-soft);
  border-radius: 999px;
  padding: 0.45rem 0.9rem;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
}
.walk-tab:hover { color: var(--ink); border-color: var(--accent); }
.walk-tab.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}
[data-theme="dark"] .walk-tab.active { color: #0a0c0e; }

.walk-progress {
  height: 4px;
  background: var(--bg-muted);
  border-radius: 99px;
  overflow: hidden;
  margin-bottom: 0.65rem;
}
.walk-progress span {
  display: block;
  height: 100%;
  background: var(--accent);
  border-radius: 99px;
  transition: width 0.25s ease;
}
.walk-meta {
  font-size: 0.85rem;
  color: var(--ink-faint);
  margin: 0 0 0.85rem;
}
.walk-meta strong { color: var(--ink); }
.walk-meta em { font-style: normal; color: var(--accent); font-weight: 500; }

.walk-steps {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 0.65rem;
}
.walk-step {
  display: grid;
  grid-template-columns: auto 1fr;
  gap: 0.75rem;
  padding: 0.85rem 0.95rem;
  border-radius: var(--radius-sm);
  background: var(--bg-muted);
  border: 1px solid transparent;
  opacity: 0.72;
  transition: opacity 0.2s, border-color 0.2s, background 0.2s;
}
.walk-step.done { opacity: 0.85; }
.walk-step.current {
  opacity: 1;
  background: var(--bg-elev);
  border-color: color-mix(in srgb, var(--accent) 45%, var(--line));
  box-shadow: 0 4px 16px color-mix(in srgb, var(--accent) 12%, transparent);
}
.walk-step-num {
  width: 28px; height: 28px;
  border-radius: 50%;
  display: grid; place-items: center;
  font-size: 0.78rem; font-weight: 700;
  background: var(--accent-soft);
  color: var(--accent);
  flex-shrink: 0;
}
.walk-step.current .walk-step-num {
  background: var(--accent);
  color: #fff;
}
[data-theme="dark"] .walk-step.current .walk-step-num { color: #0a0c0e; }
.walk-step.done .walk-step-num {
  background: color-mix(in srgb, var(--accent) 25%, var(--bg-muted));
}
.walk-step-title {
  margin: 0 0 0.35rem;
  font-size: 0.98rem;
  font-family: var(--serif);
  font-weight: 600;
  color: var(--ink);
}
.walk-step-explain {
  margin: 0;
  font-size: 0.92rem;
  color: var(--ink-soft);
  line-height: 1.55;
}
.walk-math {
  margin-top: 0.55rem;
  padding: 0.55rem 0.65rem;
  background: var(--bg-elev);
  border-radius: 8px;
  border: 1px solid var(--line);
  overflow-x: auto;
  text-align: center;
}
.walk-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-top: 1rem;
}
.walk-nav .btn:disabled {
  opacity: 0.4;
  cursor: not-allowed;
  transform: none;
}

.walk-side {
  background: var(--bg-muted);
  border-radius: var(--radius-sm);
  padding: 0.85rem;
  border: 1px solid var(--line);
  position: sticky;
  top: calc(var(--header-h) + 0.75rem);
}
.walk-side .label {
  font-size: 0.72rem; font-weight: 600; letter-spacing: 0.05em;
  text-transform: uppercase; color: var(--ink-faint); margin: 0 0 0.45rem;
}
.fn-plot {
  width: 100%;
  height: 160px;
  border-radius: 8px;
  background: var(--bg-elev);
  border: 1px solid var(--line);
}
.plot-caption {
  font-size: 0.78rem;
  color: var(--ink-faint);
  margin: 0.5rem 0 0.65rem;
  line-height: 1.4;
}
.method-pills {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
}
.pill {
  font-size: 0.78rem;
  padding: 0.35rem 0.55rem;
  border-radius: 8px;
  background: var(--bg-elev);
  border: 1px solid var(--line);
  color: var(--ink-soft);
}
.walk-empty { color: var(--ink-faint); font-size: 0.9rem; }

@media (max-width: 800px) {
  .walk-layout { grid-template-columns: 1fr; }
  .walk-side { position: static; order: -1; }
}

/* ── Footer ─────────────────────────────────────────── */
.site-footer {
  border-top: 1px solid var(--line);
  padding: 1.5rem 1.25rem;
  text-align: center; color: var(--ink-faint); font-size: 0.85rem;
}
.site-footer p { margin: 0; }

/* KaTeX tweaks */
.katex-display { margin: 0.6rem 0 !important; overflow-x: auto; overflow-y: hidden; }
.fn-block .katex { font-size: 1.05em; }

/* ── Responsive ─────────────────────────────────────── */
@media (max-width: 860px) {
  .demo-layout { grid-template-columns: 1fr; }
  .demo-readout { position: static; }
  .nav-toggle { display: flex; margin-left: auto; }
  .theme-toggle { order: 2; }
  .main-nav {
    display: none; position: absolute; left: 0; right: 0; top: var(--header-h);
    background: var(--bg-elev); border-bottom: 1px solid var(--line);
    flex-direction: column; padding: 0.75rem; gap: 0.25rem;
  }
  .main-nav.open { display: flex; }
  .main-nav a { padding: 0.7rem 0.9rem; }
  .logo-text { display: none; }
  .header-inner { position: relative; }
}