:root {
  color-scheme: light;
  --ink: #203848;
  --muted: #6c8190;
  --line: #d9eeee;
  --mint: #e7fbf7;
  --mint-strong: #2c9f92;
  --coral: #ff7b73;
  --pink: #ffdce8;
  --yellow: #ffda68;
  --blue: #77bdff;
  --shadow: 0 18px 50px rgba(29, 73, 86, 0.12);
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: "Microsoft YaHei", "PingFang SC", "Noto Sans CJK SC", Arial, sans-serif;
  color: var(--ink);
  background:
    radial-gradient(circle at 8% 2%, rgba(255, 218, 104, 0.35), transparent 28rem),
    linear-gradient(135deg, #e7fbf7 0%, #f8fffd 46%, #fff7fb 100%);
}
button, input { font: inherit; }
button { color: inherit; }

.app-header {
  position: sticky;
  top: 0;
  z-index: 30;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  margin: 1rem clamp(1rem, 3vw, 3rem);
  padding: 1rem 1.2rem;
  border: 1px solid rgba(255,255,255,.8);
  border-radius: 1.4rem;
  background: rgba(255,255,255,.9);
  box-shadow: var(--shadow);
  backdrop-filter: blur(16px);
}
.brand {
  display: flex;
  align-items: center;
  gap: .8rem;
  border: 0;
  background: transparent;
  cursor: pointer;
  text-align: left;
}
.brand-mark {
  display: grid;
  width: 3.3rem;
  height: 3.3rem;
  place-items: center;
  border: 4px solid #fff;
  border-radius: 1rem;
  background: linear-gradient(135deg, var(--coral), #ffa664);
  color: #fff;
  font-size: 1.4rem;
  font-weight: 900;
  box-shadow: 0 12px 24px rgba(255,123,115,.26);
}
.brand strong, .brand small { display: block; line-height: 1.25; }
.brand strong { font-size: 1.35rem; }
.brand small { color: var(--muted); }
.header-stats { display: flex; flex-wrap: wrap; justify-content: flex-end; gap: .65rem; }
.header-stats span {
  min-height: 2.6rem;
  padding: .62rem .85rem;
  border-radius: .9rem;
  background: var(--mint);
  color: #35736f;
  font-weight: 800;
}
.header-stats .trial-pill {
  background: #fff1d7;
  color: #9a5b00;
}
main { width: min(118rem, 100%); margin: 0 auto; padding: 0 clamp(1rem, 3vw, 3rem) 3rem; }
.hero-panel {
  display: grid;
  grid-template-columns: minmax(0,.95fr) minmax(18rem,.85fr);
  align-items: center;
  gap: 2rem;
  margin-top: 1.4rem;
  padding: clamp(1.1rem, 3vw, 2rem);
  border-radius: 1.6rem;
  background: rgba(255,255,255,.76);
  box-shadow: var(--shadow);
}
.hero-copy { max-width: 45rem; }
.eyebrow { margin: 0 0 .55rem; color: var(--mint-strong); font-size: .92rem; font-weight: 900; }
h1,h2,h3,p { overflow-wrap: anywhere; }
h1 { margin: 0; max-width: 15ch; font-size: clamp(2.1rem, 4vw, 4.6rem); line-height: 1.06; letter-spacing: 0; }
h2,h3 { margin: 0; line-height: 1.22; letter-spacing: 0; }
.hero-copy p:last-child { margin: 1rem 0 0; color: var(--muted); font-size: 1.08rem; line-height: 1.8; }
.hero-panel img { display: block; width: 100%; aspect-ratio: 4/3; border-radius: 1.2rem; object-fit: cover; }
.grade-section, .study-app { margin-top: 1.4rem; }
.section-title { margin-bottom: 1rem; }
.grade-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0,1fr));
  gap: 1rem;
}
.grade-card, .route-card, .lesson-note, .challenge-card, .quiz-card, .report-grid article, .report-card, .mistake-card, .empty-state {
  border: 1px solid rgba(107,151,157,.14);
  border-radius: 1rem;
  background: rgba(255,255,255,.86);
  box-shadow: 0 12px 32px rgba(29,73,86,.08);
}
.grade-card {
  min-height: 12rem;
  padding: 1.1rem;
  cursor: pointer;
  text-align: left;
}
.grade-card.available:hover { transform: translateY(-2px); }
.grade-card span {
  display: grid;
  width: 3rem;
  height: 3rem;
  place-items: center;
  border-radius: .9rem;
  background: var(--yellow);
  font-size: 1.4rem;
  font-weight: 900;
}
.grade-card p, .lesson-note p, .challenge-card p, .report-card p, .mistake-card p, .mistake-card small, .lesson-item small, #lessonSubtitle, #gradeDesc {
  color: var(--muted);
  line-height: 1.68;
}
.grade-card.disabled { opacity: .62; cursor: not-allowed; }
.study-title {
  display: grid;
  grid-template-columns: auto minmax(0,1fr);
  gap: 1rem;
  align-items: center;
  margin: 1.4rem 0;
}
.study-app {
  display: grid;
  grid-template-columns: minmax(17rem,23rem) minmax(0,1fr);
  gap: 1.2rem;
  align-items: start;
}
.sidebar, .workspace {
  border-radius: 1.5rem;
  background: rgba(255,255,255,.74);
  box-shadow: var(--shadow);
}
.sidebar { position: sticky; top: 6.8rem; max-height: calc(100vh - 8rem); overflow: hidden; padding: 1rem; }
.sidebar-head input {
  width: 100%;
  min-height: 3rem;
  margin-top: 1rem;
  padding: .7rem .9rem;
  border: 1px solid var(--line);
  border-radius: .9rem;
  background: #f9ffff;
  outline: none;
}
.term-switch, .feature-tabs {
  display: flex;
  gap: .5rem;
  padding: .45rem;
  border-radius: 1rem;
  background: #eaf8f8;
}
.term-switch { margin: .9rem 0; }
.term-btn, .feature-tab, .back-home, .soft-btn, .primary-btn {
  min-height: 2.75rem;
  border: 0;
  border-radius: .8rem;
  cursor: pointer;
  font-weight: 900;
}
.term-btn, .feature-tab { flex: 1; padding: .55rem .7rem; background: transparent; color: #5b7885; }
.term-btn.is-active, .feature-tab.is-active { background: #fff; color: var(--coral); box-shadow: 0 8px 20px rgba(29,73,86,.08); }
.lesson-list { display: grid; gap: .7rem; max-height: calc(100vh - 22rem); overflow: auto; padding-right: .25rem; }
.lesson-item {
  display: grid;
  grid-template-columns: 1.6rem minmax(0,1fr);
  gap: .7rem;
  width: 100%;
  min-height: 4.9rem;
  padding: .85rem;
  border: 1px solid transparent;
  border-radius: 1rem;
  background: #fff;
  cursor: pointer;
  text-align: left;
}
.lesson-item.is-active { border-color: #ffc3d7; background: #fff2f7; }
.lesson-item strong, .lesson-item small { display: block; }
.check-dot {
  display: grid;
  width: 1.4rem;
  height: 1.4rem;
  place-items: center;
  border: 2px solid #b9c9cf;
  border-radius: .35rem;
  color: var(--mint-strong);
  font-weight: 900;
}
.workspace { padding: clamp(1rem,2.4vw,1.6rem); }
.lesson-top { display: grid; gap: .3rem; }
.back-home, .soft-btn { padding: .55rem .85rem; background: var(--mint); color: #2e756f; }
.danger { background: var(--pink); color: #a9416b; }
.feature-tabs { margin: 1.1rem 0; width: min(38rem,100%); }
.view-panel { display: none; }
.view-panel.is-active { display: block; }
.lesson-note, .challenge-card { padding: 1rem; margin-bottom: 1rem; background: #fffaf0; }
.practice-head { display: flex; align-items: center; justify-content: space-between; gap: 1rem; margin-bottom: 1rem; }
.quiz-list, .mistake-list { display: grid; gap: 1rem; }
.quiz-card { padding: 1rem; }
.question-line { display: flex; align-items: flex-start; justify-content: space-between; gap: 1rem; margin-bottom: .8rem; }
.question-line span {
  flex: 0 0 auto;
  padding: .35rem .55rem;
  border-radius: 999px;
  background: var(--mint);
  color: #33756f;
  font-size: .82rem;
  font-weight: 900;
}
.choice-grid { display: grid; grid-template-columns: repeat(3, minmax(0,1fr)); gap: .75rem; }
.choice-btn {
  display: flex;
  align-items: center;
  gap: .55rem;
  min-height: 3.2rem;
  padding: .7rem;
  border: 1px solid var(--line);
  border-radius: .9rem;
  background: #fbffff;
  cursor: pointer;
  text-align: left;
  font-weight: 800;
}
.choice-btn span { display: grid; width: 1.8rem; height: 1.8rem; place-items: center; border-radius: 50%; background: #edf8ff; color: #326f9c; }
.choice-btn.is-selected { border-color: var(--coral); background: #fff4f3; }
.feedback { min-height: 1.6rem; margin: .8rem 0 0; font-weight: 800; }
.feedback.right { color: #238269; }
.feedback.wrong { color: #b94b3d; }
.result-box { min-height: 3rem; margin: 1rem 0; padding: .8rem; border-radius: .9rem; background: #f2fbff; color: #365366; font-weight: 800; }
.primary-btn { width: fit-content; padding: .7rem 1.1rem; background: var(--ink); color: #fff; }
.challenge-actions { display: flex; flex-wrap: wrap; gap: .7rem; }
.stars { color: #d79a00; font-size: 2rem; font-weight: 900; letter-spacing: 0; }
.report-grid { display: grid; grid-template-columns: repeat(4, minmax(0,1fr)); gap: 1rem; }
.report-grid article, .report-card, .mistake-card, .empty-state { padding: 1rem; }
.report-grid span { display: block; color: var(--coral); font-size: 2.4rem; font-weight: 900; line-height: 1; }
.report-card { margin-top: 1rem; }
.mistake-head {
  display: flex;
  justify-content: space-between;
  gap: .8rem;
  align-items: center;
  margin-bottom: .9rem;
  color: var(--ink);
  font-weight: 900;
}
.mistake-head strong {
  flex: 0 0 auto;
  padding: .35rem .7rem;
  border-radius: 999px;
  background: #eaf9f6;
  color: #238269;
  font-size: .86rem;
}
.mistake-question { padding: .85rem 0; border-top: 1px solid rgba(107,151,157,.14); }
.mistake-question:first-of-type { border-top: 0; }
.mistake-question.is-done { opacity: .82; }
.choice-btn:disabled { cursor: default; opacity: .72; }
.followup-box {
  margin-top: .8rem;
  padding: .85rem;
  border-radius: .9rem;
  background: #f7fcff;
  border: 1px solid rgba(107,151,157,.16);
}
.followup-box h4 { margin: 0 0 .35rem; color: var(--ink); }
.followup-box p { margin: 0 0 .7rem; color: var(--muted); }
.site-footer { padding: 2rem 1rem 3rem; color: var(--muted); text-align: center; }
.trial-expired {
  position: fixed;
  inset: 0;
  z-index: 100;
  display: grid;
  place-items: center;
  padding: 1.5rem;
  background: rgba(244, 252, 250, .94);
  backdrop-filter: blur(10px);
}
.trial-expired[hidden] { display: none; }
.trial-expired > div {
  max-width: 28rem;
  border-radius: 1.2rem;
  padding: 2rem;
  text-align: center;
  background: #fff;
  box-shadow: var(--shadow);
  border: 1px solid rgba(107,151,157,.18);
}
.trial-expired .brand-mark { margin: 0 auto 1rem; }
.trial-expired h2 { margin: 0 0 .7rem; }
.trial-expired p { margin: 0; color: var(--muted); line-height: 1.7; }
body.trial-ended { overflow: hidden; }

@media (max-width: 1050px) {
  .hero-panel, .study-app { grid-template-columns: 1fr; }
  .sidebar { position: static; max-height: none; }
  .lesson-list { max-height: 22rem; }
  .grade-grid, .report-grid { grid-template-columns: repeat(2, minmax(0,1fr)); }
}
@media (max-width: 720px) {
  .app-header { align-items: flex-start; flex-direction: column; margin: .65rem; border-radius: 1rem; }
  .header-stats { justify-content: flex-start; }
  main { padding: 0 .65rem 2rem; }
  .hero-panel { padding: 1rem; }
  h1 { max-width: none; font-size: 2.35rem; }
  .grade-grid, .report-grid, .choice-grid { grid-template-columns: 1fr; }
  .study-title, .practice-head { align-items: stretch; grid-template-columns: 1fr; flex-direction: column; }
  .feature-tabs { display: grid; grid-template-columns: repeat(2, minmax(0,1fr)); }
  .primary-btn, .soft-btn, .back-home { width: 100%; }
  .challenge-actions { flex-direction: column; }
}