:root {
  --bg: #f7f9fc;
  --paper: #ffffff;
  --ink: #172033;
  --muted: #5d6b82;
  --line: #d8e0ec;
  --blue: #1e40af;
  --teal: #0f9f9a;
  --yellow: #f4b740;
  --coral: #ef6a5b;
  --green: #258a4b;
  --red: #be3434;
  --shadow: 0 18px 45px rgba(33, 50, 82, 0.12);
}

* { box-sizing: border-box; }

body {
  margin: 0;
  color: var(--ink);
  background:
    linear-gradient(180deg, #eef6ff 0, var(--bg) 360px),
    var(--bg);
  font-family: "Segoe UI", Arial, sans-serif;
}

button, input, select { font: inherit; }

.site-header {
  position: sticky;
  top: 0;
  z-index: 5;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 20px;
  padding: 16px clamp(18px, 4vw, 48px);
  background: rgba(255, 255, 255, 0.92);
  border-bottom: 1px solid var(--line);
  backdrop-filter: blur(14px);
}

.brand {
  color: var(--blue);
  font-weight: 800;
  text-decoration: none;
}

nav {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
}

nav a {
  color: var(--ink);
  text-decoration: none;
  padding: 9px 12px;
  border-radius: 8px;
  border: 1px solid transparent;
  font-weight: 650;
  font-size: 0.95rem;
}

nav a:hover,
nav a.active {
  border-color: #bfd0eb;
  background: #edf4ff;
  color: var(--blue);
}

.home-grid,
.lesson-shell {
  width: min(1180px, calc(100% - 32px));
  margin: 0 auto;
}

.home-grid {
  display: grid;
  grid-template-columns: 0.95fr 1.05fr;
  gap: 28px;
  align-items: stretch;
  padding: 54px 0;
}

.intro-panel,
.lesson-band,
.panel,
.lesson-card {
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 8px;
  box-shadow: var(--shadow);
}

.intro-panel {
  padding: clamp(28px, 5vw, 54px);
}

.small-label {
  margin: 0 0 12px;
  color: var(--teal);
  font-size: 0.86rem;
  font-weight: 800;
  text-transform: uppercase;
  letter-spacing: 0;
}

h1, h2, h3, p { margin-top: 0; }

h1 {
  font-size: clamp(2rem, 5vw, 4.4rem);
  line-height: 1.02;
  margin-bottom: 20px;
  letter-spacing: 0;
}

h2 {
  margin-bottom: 14px;
  font-size: clamp(1.25rem, 2vw, 1.65rem);
}

p {
  color: var(--muted);
  line-height: 1.6;
}

.lesson-picker {
  display: grid;
  gap: 18px;
}

.lesson-card {
  display: grid;
  grid-template-columns: 74px 1fr;
  gap: 18px;
  align-content: start;
  padding: 26px;
  color: inherit;
  text-decoration: none;
  transition: transform 180ms ease, border-color 180ms ease;
}

.lesson-card:hover {
  transform: translateY(-3px);
  border-color: #9eb8df;
}

.lesson-card strong,
.lesson-card small {
  display: block;
}

.lesson-card small {
  margin-top: 8px;
  color: var(--muted);
  line-height: 1.45;
}

.card-icon {
  display: grid;
  place-items: center;
  width: 62px;
  height: 62px;
  border-radius: 8px;
  background: #e8f7f6;
  color: var(--teal);
  font-size: 1.35rem;
  font-weight: 900;
}

.card-action {
  grid-column: 2;
  color: var(--blue);
  font-weight: 800;
}

.lesson-band {
  display: grid;
  grid-template-columns: 1fr minmax(280px, 430px);
  gap: 24px;
  align-items: center;
  margin: 28px 0;
  padding: clamp(24px, 4vw, 42px);
  border-left: 8px solid var(--teal);
}

.formula-spotlight {
  display: grid;
  gap: 10px;
  padding: 24px;
  border-radius: 8px;
  color: white;
  background: linear-gradient(135deg, var(--blue), #13276d);
}

.formula-spotlight > span {
  font-size: clamp(1.55rem, 3.7vw, 2.25rem);
  font-weight: 900;
}

.formula-spotlight small {
  color: #d8e4ff;
  font-weight: 700;
}

.content-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
  padding-bottom: 46px;
}

.panel {
  padding: 22px;
}

.wide { grid-column: 1 / -1; }

.panel-title-row {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  gap: 14px;
}

output {
  color: var(--blue);
  font-weight: 800;
}

.route-layout,
.graph-layout {
  display: grid;
  grid-template-columns: 330px 1fr;
  gap: 20px;
  align-items: start;
}

.controls {
  display: grid;
  gap: 15px;
}

.stage-card {
  min-width: 0;
  border: 1px solid var(--line);
  border-radius: 8px;
  background:
    linear-gradient(90deg, rgba(36, 72, 189, 0.08) 1px, transparent 1px) 0 0 / 56px 56px,
    linear-gradient(180deg, #f9fcff, #eef7ff);
  overflow: hidden;
}

canvas {
  display: block;
  width: 100%;
  min-height: 320px;
  background: transparent;
}

.button-row {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
}

.button-row button,
.primary,
.secondary {
  min-height: 42px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fff;
  color: var(--ink);
  padding: 10px 14px;
  cursor: pointer;
  font-weight: 750;
}

.trainer-next {
  margin-top: 12px;
}

.button-row button:hover,
.secondary:hover {
  border-color: #95add1;
  background: #f4f8ff;
}

.button-row button.selected {
  background: var(--blue);
  border-color: var(--blue);
  color: #fff;
}

.scale-card,
.answer,
.compare-grid > div {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #f9fbff;
}

.scale-card {
  padding: 14px;
  color: var(--muted);
  line-height: 1.5;
}

.scale-card strong {
  display: block;
  margin-bottom: 8px;
  color: var(--ink);
}

.hint {
  padding: 12px 14px;
  border-left: 4px solid var(--yellow);
  border-radius: 8px;
  background: #fff8e8;
  color: #5b4420;
  line-height: 1.5;
  font-weight: 650;
}

.compare-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
}

.compare-grid > div {
  display: grid;
  gap: 8px;
  padding: 16px;
}

.compare-grid strong {
  color: var(--blue);
  font-size: 1.18rem;
}

.compare-grid span {
  color: var(--muted);
}

.select-label {
  display: grid;
  gap: 8px;
  color: var(--muted);
  font-weight: 750;
}

.select-label select,
.practice-row input {
  width: 100%;
  min-height: 44px;
  border-radius: 8px;
  border: 1px solid #c5d1e1;
  padding: 10px 12px;
  color: var(--ink);
  background: white;
}

.data-table {
  display: grid;
  border: 1px solid var(--line);
  border-radius: 8px;
  overflow: hidden;
}

.data-table div {
  display: grid;
  grid-template-columns: 1fr 1fr;
}

.data-table span {
  padding: 9px 11px;
  border-bottom: 1px solid var(--line);
  background: #fff;
  font-weight: 750;
}

.data-table div:first-child span {
  background: #edf4ff;
  color: var(--blue);
}

.data-table div:last-child span {
  border-bottom: 0;
}

.data-table span + span {
  border-left: 1px solid var(--line);
}

.primary {
  background: var(--blue);
  border-color: var(--blue);
  color: white;
}

.primary:hover { background: #163487; }

.secondary { background: #edf4ff; }

.answer {
  margin-top: 14px;
  padding: 14px;
  min-height: 52px;
  line-height: 1.55;
}

.answer.good {
  border-color: #a5d8b7;
  background: #f0fbf4;
  color: var(--green);
}

.answer.bad {
  border-color: #f0b5b5;
  background: #fff4f4;
  color: var(--red);
}

.practice-row {
  display: grid;
  grid-template-columns: 1fr auto auto auto;
  gap: 10px;
  align-items: center;
}

@media (max-width: 900px) {
  .site-header,
  .home-grid,
  .lesson-band,
  .content-grid,
  .route-layout,
  .graph-layout {
    grid-template-columns: 1fr;
  }

  .site-header,
  .panel-title-row {
    display: grid;
    align-items: start;
  }

  .content-grid,
  .home-grid {
    gap: 14px;
  }

  .compare-grid {
    grid-template-columns: 1fr;
  }
}

@media (max-width: 560px) {
  .lesson-card,
  .practice-row {
    grid-template-columns: 1fr;
  }

  .card-action {
    grid-column: 1;
  }

  h1 { font-size: 2.15rem; }
  canvas { min-height: 280px; }
}