/* Chapter entry: reader paper surfaces and existing Mathos artwork. */
body[data-page="chapter-activities"] {
  --rail-width: 80px;
  --activity-bg: #f1f4f6;
  --activity-paper: #fff;
  --activity-ink: #263447;
  --activity-muted: #586477;
  --activity-line: #d8dfed;
  --activity-teal: #156979;
  --activity-violet: #6f3db6;
  --course-surface: var(--activity-paper);
  --course-ink: var(--activity-ink);
  --course-muted: var(--activity-muted);
  --course-border: var(--activity-line);
  --course-accent: var(--activity-teal);
  --course-heading: var(--activity-teal);

  margin: 0;
  min-height: 100svh;
  color: var(--activity-ink);
  background: var(--activity-bg);
  font-family: var(--font-ui);
}

body[data-page="chapter-activities"][data-theme="dark"] {
  --activity-bg: #0d1523;
  --activity-paper: #111a2c;
  --activity-ink: #e7edf8;
  --activity-muted: #a5b3c8;
  --activity-line: #34415b;
  --activity-teal: #78d7e0;
  --activity-violet: #c6a7ff;
}

.chapter-activity-shell {
  display: grid;
  grid-template-columns: var(--rail-width) minmax(0, 1fr);
  min-height: 100svh;
}

.chapter-activity-reader { min-width: 0; }

.chapter-activity-main {
  box-sizing: border-box;
  width: min(100%, 1020px);
  margin: 0 auto;
  padding: 38px 44px 100px;
}

.chapter-activity-heading {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  min-height: 170px;
  margin-bottom: 28px;
}

.chapter-activity-heading > div { min-width: 0; }

.chapter-activity-number {
  display: inline-block;
  margin: 0 0 16px;
  padding: 7px 13px;
  border: 1px solid var(--activity-line);
  border-radius: 10px;
  color: var(--activity-teal);
  background: var(--activity-paper);
  font-size: 0.82rem;
  font-weight: 750;
}

.chapter-activity-heading h1 {
  max-width: 650px;
  margin: 0;
  color: var(--activity-ink);
  font: 800 clamp(1.5rem, 2.8vw, 2.1rem)/1.25 var(--font-ui);
  letter-spacing: -0.035em;
  text-wrap: balance;
}

.chapter-activity-character {
  display: block;
  flex: 0 0 170px;
  object-fit: contain;
  filter: drop-shadow(0 8px 14px rgb(22 86 100 / 14%));
  mask-image: linear-gradient(#000 82%, transparent);
}

.chapter-activity-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 26px;
}

.chapter-activity-card {
  --activity-accent: var(--activity-teal);

  position: relative;
  display: flex;
  flex-direction: column;
  min-width: 0;
  padding: 30px;
  border: 1px solid color-mix(in srgb, var(--activity-accent) 32%, var(--activity-line));
  border-radius: 24px;
  color: var(--activity-ink);
  background:
    linear-gradient(
      145deg,
      color-mix(in srgb, var(--activity-accent) 5%, var(--activity-paper)),
      var(--activity-paper) 65%
    );
  box-shadow: -6px 8px 14px -8px rgb(22 38 60 / 20%), 0 4px 0 var(--activity-line);
  text-decoration: none;
  transition: transform 180ms ease, box-shadow 180ms ease, border-color 180ms ease;
}

.chapter-activity-card--qcm { --activity-accent: var(--activity-violet); }

.chapter-activity-card-top {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 12px;
  margin-bottom: 25px;
}

.chapter-activity-icon {
  display: grid;
  place-items: center;
  width: 86px;
  height: 86px;
  border: 1px solid color-mix(in srgb, var(--activity-accent) 20%, var(--activity-paper));
  border-radius: 23px;
  color: var(--activity-accent);
  background: color-mix(in srgb, var(--activity-accent) 10%, var(--activity-paper));
  box-shadow: 0 4px 0 color-mix(in srgb, var(--activity-accent) 15%, var(--activity-paper));
  transform: rotate(-5deg);
  transition: transform 180ms ease;
}

.chapter-activity-icon .mathos-nav-symbol { width: 52px; height: 52px; color: inherit; }
.chapter-activity-card--qcm .chapter-activity-icon { transform: rotate(5deg); }

.chapter-activity-tag {
  padding: 6px 10px;
  border-radius: 8px;
  color: var(--activity-accent);
  background: color-mix(in srgb, var(--activity-accent) 8%, var(--activity-paper));
  font-size: 0.75rem;
  font-weight: 700;
}

.chapter-activity-card h2 {
  margin: 0 0 22px;
  color: var(--activity-ink);
  font: 800 1.65rem/1.2 var(--font-ui);
  letter-spacing: -0.025em;
}

.chapter-activity-card p {
  flex: 1;
  margin: 0 0 26px;
  color: var(--activity-muted);
  font-size: 0.92rem;
  line-height: 1.6;
}

.chapter-activity-action {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  box-sizing: border-box;
  width: 100%;
  min-height: 50px;
  padding: 12px 18px;
  border: 1px solid color-mix(in srgb, var(--activity-accent) 30%, var(--activity-paper));
  border-radius: 12px;
  color: var(--activity-accent);
  background: color-mix(in srgb, var(--activity-accent) 10%, var(--activity-paper));
  box-shadow: 0 3px 0 color-mix(in srgb, var(--activity-accent) 38%, var(--activity-paper));
  font: 750 1rem/1.3 var(--font-ui);
}

.chapter-activity-action > span { font-size: 1.4rem; line-height: 1; }

.chapter-activity-action:disabled {
  color: var(--activity-muted);
  background: var(--activity-bg);
  border-color: var(--activity-line);
  box-shadow: none;
  cursor: not-allowed;
}

.chapter-activity-card:focus-visible {
  outline: 3px solid var(--activity-accent);
  outline-offset: 6px;
}

@media (hover: hover) and (pointer: fine) {
  a.chapter-activity-card:hover {
    color: var(--activity-ink);
    border-color: var(--activity-accent);
    transform: translateY(-5px);
    box-shadow:
      -6px 12px 22px -8px rgb(22 38 60 / 20%),
      0 4px 0 color-mix(in srgb, var(--activity-accent) 50%, var(--activity-line));
  }

  a.chapter-activity-card:hover .chapter-activity-icon { transform: rotate(0) scale(1.06); }
}

a.chapter-activity-card:active { transform: translateY(2px); }

@media (max-width: 799px) {
  .chapter-activity-shell { grid-template-columns: minmax(0, 1fr); }
  .chapter-activity-main { padding: 24px 20px 110px; }
  .chapter-activity-heading { gap: 8px; min-height: 110px; margin-bottom: 24px; }
  .chapter-activity-character { flex-basis: 100px; width: 100px; height: 130px; object-fit: cover; }
  .chapter-activity-grid { grid-template-columns: minmax(0, 1fr); gap: 22px; }
  .chapter-activity-card { padding: 22px; }
  .chapter-activity-card-top { margin-bottom: 18px; }
  .chapter-activity-icon { width: 64px; height: 64px; border-radius: 18px; }
  .chapter-activity-icon .mathos-nav-symbol { width: 42px; height: 42px; }
  .chapter-activity-card h2 { font-size: 1.4rem; }
  .chapter-activity-card p { margin-bottom: 20px; }
}

@media (prefers-reduced-motion: reduce) {
  .chapter-activity-card,
  .chapter-activity-icon { transition: none; }

  a.chapter-activity-card:hover,
  a.chapter-activity-card:active,
  a.chapter-activity-card:hover .chapter-activity-icon { transform: none; }
}