* { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  background: linear-gradient(135deg, #f0f4ff 0%, #fafffe 50%, #f5f0ff 100%);
  color: #1a1a2e;
  min-height: 100vh;
}

/* ── Header ── */
header {
  background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
  padding: 2.5rem 1rem 2rem;
  text-align: center;
}
header h1 { font-size: 1.9rem; font-weight: 700; color: #fff; letter-spacing: .02em; }
.subtitle { color: rgba(255,255,255,.82); margin: .4rem 0 1.4rem; font-size: .95rem; }

.search-box input {
  width: 100%; max-width: 520px;
  padding: .75rem 1.2rem;
  border: 2px solid rgba(255,255,255,.4);
  border-radius: 2rem;
  font-size: 1rem;
  outline: none;
  background: rgba(255,255,255,.92);
  transition: border-color .2s, box-shadow .2s;
}
.search-box input:focus {
  border-color: #fff;
  box-shadow: 0 0 0 3px rgba(255,255,255,.3);
}

/* ── Main ── */
main { max-width: 1100px; margin: 0 auto; padding: 2rem 1rem; }
.hidden { display: none !important; }
.section-title { font-size: 1.1rem; color: #6c757d; margin-bottom: 1.2rem; }

/* ── Category Grid ── */
.category-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 1rem;
}
.category-card {
  background: #fff;
  border-radius: 14px;
  padding: 1.5rem;
  cursor: pointer;
  border: 2px solid transparent;
  border-left: 5px solid var(--cat-color);
  box-shadow: 0 2px 12px rgba(0,0,0,.06);
  transition: transform .15s, box-shadow .15s;
  display: flex; align-items: center; gap: 1rem;
}
.category-card:hover {
  transform: translateY(-4px);
  box-shadow: 0 10px 28px rgba(0,0,0,.12);
}
.category-card .icon { font-size: 2rem; }
.category-card h3 { font-size: 1.05rem; margin-bottom: .2rem; }
.category-card p { font-size: .82rem; color: #6c757d; }

/* ── Back button ── */
.back-btn {
  background: none; border: none; cursor: pointer;
  color: #4A90D9; font-size: .95rem; padding: .4rem 0;
  margin-bottom: 1rem; display: inline-block;
}
.back-btn:hover { text-decoration: underline; }

/* ── Terms Grid ── */
.terms-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: 1rem;
  margin-bottom: 2rem;
}

/* ── Flip Card ── */
.flip-card { height: 160px; perspective: 800px; cursor: pointer; }
.flip-card-inner {
  position: relative; width: 100%; height: 100%;
  transform-style: preserve-3d;
  transition: transform .45s ease;
}
.flip-card.flipped .flip-card-inner { transform: rotateY(180deg); }

.flip-front, .flip-back {
  position: absolute; width: 100%; height: 100%;
  backface-visibility: hidden;
  border-radius: 10px;
  padding: 1rem;
  display: flex; flex-direction: column; justify-content: center;
}
.flip-front {
  background: #fff;
  border: 1px solid #eef0f5;
  box-shadow: 0 2px 10px rgba(0,0,0,.06);
  text-align: center;
}
.flip-card:hover .flip-front {
  box-shadow: 0 6px 20px rgba(0,0,0,.1);
}
.flip-front .en { font-size: 1rem; font-weight: 700; margin-bottom: .3rem; }
.flip-front .zh { font-size: .85rem; color: #6c757d; }
.flip-front .hint { font-size: .7rem; color: #adb5bd; margin-top: .6rem; }

.flip-back {
  background: var(--cat-color, #4A90D9);
  color: #fff;
  transform: rotateY(180deg);
}
.flip-back .def { font-size: .82rem; line-height: 1.5; margin-bottom: .5rem; }
.flip-back .analogy { font-size: .78rem; opacity: .88; font-style: italic; }

/* ── Test bar ── */
.test-bar { text-align: center; padding: 1rem 0 2rem; }
.test-btn {
  background: #1a1a2e; color: #fff;
  border: none; border-radius: 2rem;
  padding: .75rem 2.5rem; font-size: 1rem;
  cursor: pointer; transition: background .2s;
}
.test-btn:hover { background: #2d2d4e; }

/* ── Quiz ── */
.quiz-container { max-width: 640px; margin: 0 auto; }
.quiz-progress { color: #6c757d; font-size: .9rem; margin-bottom: 1.5rem; }
.quiz-question {
  background: #fff; border-radius: 12px;
  padding: 1.5rem; margin-bottom: 1rem;
  border: 1px solid #e9ecef;
}
.quiz-question .q-label {
  font-size: .75rem; color: #6c757d; margin-bottom: .5rem; text-transform: uppercase;
}
.quiz-question .q-text { font-size: 1.2rem; font-weight: 600; }

.quiz-options { display: grid; gap: .6rem; }
.quiz-option {
  background: #fff; border: 2px solid #e9ecef;
  border-radius: 8px; padding: .85rem 1rem;
  cursor: pointer; font-size: .95rem;
  transition: border-color .15s, background .15s;
  text-align: left;
}
.quiz-option:hover:not(:disabled) { border-color: #4A90D9; background: #f0f7ff; }
.quiz-option.correct { border-color: #27AE60; background: #eafaf1; color: #1e8449; }
.quiz-option.wrong   { border-color: #E74C3C; background: #fdf2f2; color: #c0392b; }
.quiz-option:disabled { cursor: default; }

/* ── Result ── */
.quiz-result {
  background: #fff; border-radius: 12px;
  padding: 2rem; text-align: center;
  border: 1px solid #e9ecef;
}
.quiz-result .score { font-size: 3rem; font-weight: 700; color: #1a1a2e; }
.quiz-result .score-label { color: #6c757d; margin-bottom: 1.5rem; }
.wrong-list { text-align: left; margin-top: 1.5rem; }
.wrong-list h3 { font-size: 1rem; margin-bottom: .8rem; color: #E74C3C; }
.wrong-item {
  background: #fdf2f2; border-radius: 8px;
  padding: .75rem 1rem; margin-bottom: .5rem;
  font-size: .88rem; line-height: 1.6;
}
.rank-info {
  font-size: .9rem; color: #6c757d;
  margin-bottom: 1rem;
}
.rank-info strong { color: #5a4fcf; font-size: 1.1rem; }

  font-size: 1rem; color: #5a4fcf;
  margin: .5rem 0 1.2rem;
  font-weight: 500;
}
  background: #1a1a2e; color: #fff;
  border: none; border-radius: 2rem;
  padding: .65rem 2rem; font-size: .95rem;
  cursor: pointer; margin-top: 1.5rem;
}
.retry-btn:hover { background: #2d2d4e; }