:root {
  --ink: #102a43;
  --muted: #52677d;
  --blue: #0b5cad;
  --blue-dark: #073b70;
  --blue-soft: #eaf4ff;
  --orange: #f28c28;
  --orange-soft: #fff3e6;
  --green: #0f7b6c;
  --red: #b42318;
  --surface: #ffffff;
  --surface-alt: #f4f8fc;
  --line: #d8e3ee;
  --shadow: 0 18px 50px rgba(16, 42, 67, 0.11);
  --radius: 20px;
  --content: 1180px;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  color: var(--ink);
  background: #f8fbfe;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  line-height: 1.65;
}

img, canvas { max-width: 100%; }

a { color: var(--blue); }

a:focus-visible,
button:focus-visible,
input:focus-visible {
  outline: 3px solid rgba(242, 140, 40, 0.55);
  outline-offset: 3px;
}

.skip-link {
  position: fixed;
  z-index: 100;
  top: 12px;
  left: 12px;
  transform: translateY(-160%);
  padding: 10px 14px;
  border-radius: 10px;
  background: var(--ink);
  color: #fff;
}

.skip-link:focus { transform: none; }

.site-header {
  position: sticky;
  z-index: 20;
  top: 0;
  border-bottom: 1px solid rgba(216, 227, 238, 0.9);
  background: rgba(255, 255, 255, 0.94);
  backdrop-filter: blur(14px);
}

.nav-shell,
.page-shell {
  width: min(calc(100% - 32px), var(--content));
  margin-inline: auto;
}

.nav-shell {
  display: flex;
  min-height: 72px;
  align-items: center;
  justify-content: space-between;
  gap: 20px;
}

.brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  color: var(--ink);
  font-weight: 800;
  text-decoration: none;
}

.brand img:not(.brand-logo) { width: 46px; height: 46px; object-fit: contain; }

.nav-shell.index-logo-nav {
  min-height: 100px;
  padding: 10px 0;
}

.brand-logo {
  display: block;
  width: auto;
  height: 80px;
}

.nav-links { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }

.nav-links a {
  padding: 9px 12px;
  border-radius: 999px;
  color: var(--ink);
  font-size: 0.95rem;
  text-decoration: none;
}

.nav-links a:hover { background: var(--blue-soft); color: var(--blue-dark); }

.hero {
  overflow: hidden;
  border-bottom: 1px solid var(--line);
  background:
    radial-gradient(circle at 85% 25%, rgba(242, 140, 40, 0.16), transparent 28%),
    radial-gradient(circle at 10% 15%, rgba(11, 92, 173, 0.14), transparent 32%),
    linear-gradient(135deg, #f5faff, #fffaf4);
}

.hero-inner { padding: clamp(54px, 8vw, 96px) 0; }

.eyebrow {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 14px;
  padding: 6px 12px;
  border: 1px solid #bfd9f4;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.74);
  color: var(--blue-dark);
  font-size: 0.86rem;
  font-weight: 800;
  letter-spacing: 0.04em;
  text-transform: uppercase;
}

h1, h2, h3 { line-height: 1.18; }

h1 {
  max-width: 900px;
  margin: 0;
  font-size: clamp(2.25rem, 5vw, 4.8rem);
  letter-spacing: -0.035em;
}

h2 { margin: 0 0 16px; font-size: clamp(1.7rem, 3vw, 2.65rem); }
h3 { margin: 0 0 10px; font-size: 1.2rem; }

.lead {
  max-width: 780px;
  margin: 22px 0 0;
  color: var(--muted);
  font-size: clamp(1.05rem, 2vw, 1.28rem);
}

.actions { display: flex; gap: 12px; flex-wrap: wrap; margin-top: 28px; }

.button {
  display: inline-flex;
  min-height: 46px;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 11px 18px;
  border: 1px solid transparent;
  border-radius: 12px;
  background: var(--blue);
  color: #fff;
  font: inherit;
  font-weight: 800;
  text-decoration: none;
  cursor: pointer;
}

.button:hover { background: var(--blue-dark); }
.button.secondary { border-color: var(--line); background: #fff; color: var(--ink); }
.button.secondary:hover { border-color: #98b9d9; background: var(--blue-soft); }
.button.orange { background: var(--orange); color: #241506; }
.button.orange:hover { background: #d97513; color: #fff; }

main section { padding: clamp(46px, 6vw, 78px) 0; }

.section-heading { max-width: 760px; margin-bottom: 28px; }
.section-heading p { margin: 0; color: var(--muted); font-size: 1.08rem; }

.grid { display: grid; gap: 22px; }
.grid.two { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.grid.three { grid-template-columns: repeat(3, minmax(0, 1fr)); }
.grid.four { grid-template-columns: repeat(4, minmax(0, 1fr)); }

.card {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: 0 10px 30px rgba(16, 42, 67, 0.06);
}

.card-body { padding: clamp(20px, 3vw, 30px); }
.card p:last-child { margin-bottom: 0; }

.topic-card {
  display: flex;
  min-height: 100%;
  flex-direction: column;
  overflow: hidden;
}

.topic-card .card-body { display: flex; flex: 1; flex-direction: column; }
.topic-card .button { align-self: flex-start; margin-top: auto; }

.topic-actions {
  display: flex;
  gap: 10px;
  align-items: stretch;
  flex-wrap: wrap;
  margin-top: auto;
}

.topic-actions .button {
  flex: 1 1 150px;
  margin-top: 0;
}

.topic-kicker {
  margin-bottom: 10px;
  color: var(--green);
  font-size: 0.82rem;
  font-weight: 800;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.app-layout {
  display: grid;
  grid-template-columns: minmax(250px, 340px) minmax(0, 1fr);
  gap: 24px;
  align-items: start;
}

@media (min-width: 901px) {
  [data-sine-model] .app-layout {
    grid-template-columns: minmax(420px, 460px) minmax(0, 1fr);
  }

  [data-sine-model] .formula-box {
    white-space: nowrap;
  }
}

.controls { position: sticky; top: 94px; }

.control-group { margin-bottom: 20px; }

.control-label {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 8px;
  font-weight: 800;
}

.control-value {
  min-width: 68px;
  padding: 3px 8px;
  border-radius: 8px;
  background: var(--blue-soft);
  color: var(--blue-dark);
  text-align: center;
  font-variant-numeric: tabular-nums;
}

input[type="range"] { width: 100%; accent-color: var(--blue); }

input[type="number"] {
  width: 100%;
  padding: 11px 12px;
  border: 1px solid #b8cadb;
  border-radius: 10px;
  background: #fff;
  color: var(--ink);
  font: inherit;
}

.model-form-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 14px;
}

.validation-message {
  min-height: 28px;
  margin: 12px 0 0;
  color: var(--red);
  font-weight: 800;
}

.canvas-card { padding: 14px; }

.graph-canvas {
  display: block;
  width: 100%;
  min-height: 440px;
  border-radius: 14px;
  background: #fff;
  touch-action: none;
}

.unit-circle-canvas {
  display: block;
  width: 100%;
  min-height: 560px;
  border-radius: 14px;
  background: #fff;
  touch-action: none;
}

.preset-grid {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 8px;
  margin-top: 18px;
}

.preset-button {
  min-height: 38px;
  padding: 7px 6px;
  border: 1px solid var(--line);
  border-radius: 9px;
  background: #fff;
  color: var(--ink);
  font: inherit;
  font-weight: 800;
  cursor: pointer;
}

.preset-button:hover,
.preset-button[aria-pressed="true"] {
  border-color: #98b9d9;
  background: var(--blue-soft);
  color: var(--blue-dark);
}

.toggle-list { display: grid; gap: 9px; margin-top: 18px; }
.toggle-list label { display: flex; gap: 9px; align-items: center; cursor: pointer; }
.toggle-list input { width: 18px; height: 18px; accent-color: var(--blue); }

.formula-box {
  margin-top: 14px;
  padding: 14px 16px;
  border-left: 4px solid var(--orange);
  border-radius: 10px;
  background: var(--orange-soft);
  font-family: "Cambria Math", Cambria, Georgia, serif;
  font-size: clamp(1.05rem, 2vw, 1.35rem);
  overflow-wrap: anywhere;
}

.formula-label {
  margin-top: 16px;
  color: var(--muted);
  font-size: 0.78rem;
  font-weight: 850;
  letter-spacing: 0.06em;
  text-transform: uppercase;
}

.formula-label + .formula-box { margin-top: 6px; }

.formula-box.normal-form {
  border-left-color: var(--blue);
  background: var(--surface-alt);
}

.stat-grid {
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 10px;
  margin-top: 14px;
}

.stat {
  padding: 12px;
  border-radius: 12px;
  background: var(--surface-alt);
}

.stat span { display: block; color: var(--muted); font-size: 0.8rem; }
.stat strong { font-variant-numeric: tabular-nums; }

.challenge { margin-top: 24px; border-top: 1px solid var(--line); padding-top: 22px; }
.challenge-options { display: grid; gap: 8px; margin: 14px 0; }
.challenge-options label {
  display: flex;
  gap: 10px;
  align-items: center;
  padding: 9px 11px;
  border: 1px solid var(--line);
  border-radius: 10px;
  cursor: pointer;
}

.feedback { min-height: 28px; margin: 12px 0 0; font-weight: 800; }
.feedback.success { color: var(--green); }
.feedback.error { color: var(--red); }

.definition-list { display: grid; gap: 14px; margin: 0; }
.definition-list div { padding: 18px; border-radius: 14px; background: var(--surface-alt); }
.definition-list dt { font-weight: 850; }
.definition-list dd { margin: 5px 0 0; color: var(--muted); }

.callout {
  padding: 20px 22px;
  border: 1px solid #f4c88f;
  border-radius: 16px;
  background: var(--orange-soft);
}

.callout strong { color: #7a3d00; }

.prose { max-width: 880px; }
.prose p { margin: 0 0 18px; }
.prose li + li { margin-top: 8px; }

.step-list { counter-reset: step; display: grid; gap: 18px; padding: 0; list-style: none; }
.step-list li { position: relative; padding: 20px 20px 20px 74px; border: 1px solid var(--line); border-radius: 16px; background: #fff; }
.step-list li::before {
  counter-increment: step;
  content: counter(step);
  position: absolute;
  top: 18px;
  left: 20px;
  display: grid;
  width: 38px;
  height: 38px;
  place-items: center;
  border-radius: 50%;
  background: var(--blue);
  color: #fff;
  font-weight: 900;
}

.site-footer { margin-top: 30px; padding: 36px 0; border-top: 1px solid var(--line); background: #eff5fa; color: var(--muted); }
.footer-inner { display: flex; justify-content: space-between; gap: 20px; flex-wrap: wrap; }
.footer-links { display: flex; gap: 14px; flex-wrap: wrap; }

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

@media (max-width: 900px) {
  .grid.three, .grid.four { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .app-layout { grid-template-columns: 1fr; }
  .controls { position: static; }
  .stat-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }
}

@media (max-width: 680px) {
  .nav-shell { align-items: flex-start; padding: 12px 0; }
  .nav-links { justify-content: flex-end; }
  .nav-links a:not(.nav-essential) { display: none; }
  .grid.two, .grid.three, .grid.four { grid-template-columns: 1fr; }
  .graph-canvas { min-height: 340px; }
  .unit-circle-canvas { min-height: 700px; }
  .actions .button { width: 100%; }
  .model-form-grid { grid-template-columns: 1fr; }
}

@media (max-width: 768px) {
  .brand-logo { height: 65px; }
}

@media (max-width: 480px) {
  .brand-logo { height: 52px; }
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { scroll-behavior: auto !important; }
}

@media print {
  .site-header, .site-footer, .actions, .challenge, .controls { display: none !important; }
  body { background: #fff; }
  .hero { background: #fff; }
  .hero-inner, main section { padding: 24px 0; }
  .card { break-inside: avoid; box-shadow: none; }
}