/* ── Chapter summary pages — shared stylesheet ────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --crimson:  #9B1C1C;
  --gold:     #C8A44A;
  --dark:     #1A1A2E;
  --mid:      #2D3561;
  --light-bg: #F7F4EE;
  --card-bg:  #FFFFFF;
  --text:     #2C2C2C;
  --muted:    #6B6B6B;
  --radius:   10px;
  --shadow:   0 2px 14px rgba(0,0,0,.10);
}

html { scroll-behavior: smooth; }

body {
  font-family: 'Sarabun', 'Source Sans 3', sans-serif;
  background: var(--light-bg);
  color: var(--text);
  line-height: 1.8;
  font-size: 16px;
}

a { color: var(--crimson); text-decoration: none; }
a:hover { text-decoration: underline; }

/* ── Header ── */
.ch-header {
  background: linear-gradient(135deg, var(--dark) 0%, var(--mid) 60%, var(--crimson) 100%);
  color: #fff;
  padding: 2.5rem 1.5rem 2rem;
  position: relative;
}
.back {
  display: inline-block;
  color: rgba(255,255,255,.75);
  font-size: .82rem;
  margin-bottom: 1rem;
  transition: color .15s;
}
.back:hover { color: #fff; text-decoration: none; }
.ch-num {
  display: inline-block;
  background: var(--gold);
  color: var(--dark);
  font-size: .78rem;
  font-weight: 700;
  letter-spacing: .1em;
  text-transform: uppercase;
  padding: .25rem .85rem;
  border-radius: 20px;
  margin-bottom: .75rem;
}
.ch-header h1 {
  font-size: clamp(1.5rem, 4vw, 2.2rem);
  font-weight: 700;
  line-height: 1.3;
}
.ch-en {
  margin-top: .35rem;
  font-size: .95rem;
  opacity: .8;
  font-weight: 300;
}

/* ── Layout ── */
.container {
  max-width: 820px;
  margin: 0 auto;
  padding: 2rem 1.25rem 3rem;
  display: flex;
  flex-direction: column;
  gap: 2rem;
}

/* ── Cards ── */
.card {
  background: var(--card-bg);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 1.5rem 1.75rem;
}
.card h2 {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--dark);
  border-left: 4px solid var(--crimson);
  padding-left: .65rem;
  margin-bottom: 1rem;
}
.card h2 .en {
  font-size: .78rem;
  font-weight: 400;
  color: var(--muted);
  margin-left: .4rem;
}

/* ── Topic list ── */
.topic-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: .6rem;
}
.topic-list li {
  padding-left: 1.1rem;
  position: relative;
  font-size: .95rem;
}
.topic-list li::before {
  content: '▸';
  position: absolute;
  left: 0;
  color: var(--crimson);
  font-size: .75rem;
  top: .18rem;
}
.topic-list li strong {
  color: var(--dark);
}
.topic-list li .note {
  display: block;
  font-size: .82rem;
  color: var(--muted);
  margin-top: .1rem;
}

/* ── Formula box ── */
.formula-box {
  background: #FFF9ED;
  border-left: 3px solid var(--gold);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: .9rem 1.1rem;
  margin-top: .5rem;
  font-size: .93rem;
}
.formula-box + .formula-box { margin-top: .75rem; }
.formula-box .label {
  font-size: .78rem;
  font-weight: 700;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: .06em;
  margin-bottom: .3rem;
}
.formula-box p { margin-top: .25rem; font-size: .9rem; }

/* ── Term chips ── */
.terms {
  display: flex;
  flex-wrap: wrap;
  gap: .45rem;
}
.term {
  background: var(--light-bg);
  border: 1px solid #ddd;
  border-radius: 20px;
  padding: .2rem .75rem;
  font-size: .8rem;
  color: var(--text);
  white-space: nowrap;
}
.term .sep { color: var(--muted); margin: 0 .2rem; }
.term .en-word { color: var(--mid); font-weight: 600; }

/* ── Summary highlight ── */
.summary-box {
  background: #F0F4FF;
  border-left: 3px solid var(--mid);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: .9rem 1.1rem;
  font-size: .93rem;
  line-height: 1.7;
}

/* ── Footer ── */
footer {
  background: var(--dark);
  color: rgba(255,255,255,.6);
  text-align: center;
  padding: 1.5rem 1rem;
  font-size: .78rem;
}
footer a { color: var(--gold); }

@media (max-width: 540px) {
  .ch-header { padding: 2rem 1rem 1.5rem; }
  .container  { padding: 1.5rem 1rem 2rem; }
}