:root {
  --bg: #f6f7f9;
  --surface: #ffffff;
  --ink: #1a2233;
  --ink-soft: #5a6478;
  --line: #e3e7ee;
  --accent: #3b5bdb;
  --accent-ink: #ffffff;
  --good: #2b8a3e;
  --warn: #e8590c;
  --bad: #c92a2a;
  --good-bg: #ebfbee;
  --warn-bg: #fff4e6;
  --bad-bg: #fff5f5;
  --radius: 10px;
  --shadow: 0 1px 3px rgba(16, 24, 40, 0.08);
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #12151c;
    --surface: #1b202b;
    --ink: #e8ecf4;
    --ink-soft: #9aa4b8;
    --line: #2b3242;
    --accent: #748ffc;
    --accent-ink: #0e1330;
    --good: #69db7c;
    --warn: #ffa94d;
    --bad: #ff8787;
    --good-bg: #16301c;
    --warn-bg: #33230f;
    --bad-bg: #331414;
    --shadow: 0 1px 3px rgba(0, 0, 0, 0.4);
  }
}

* { box-sizing: border-box; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font: 16px/1.6 -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}
.wrap { max-width: 860px; margin: 0 auto; padding: 0 1.25rem; }

.site-header { background: var(--surface); border-bottom: 1px solid var(--line); padding: 1rem 0; }
.site-header .wrap { display: flex; align-items: baseline; gap: 1rem; flex-wrap: wrap; }
.logo { font-size: 1.4rem; font-weight: 800; letter-spacing: -0.02em; }
.logo em { color: var(--accent); font-style: normal; }
.tagline { color: var(--ink-soft); font-size: 0.9rem; }

.stepper {
  display: flex; gap: 0.4rem; list-style: none;
  padding: 1rem 0 0; margin: 0; flex-wrap: wrap;
  counter-reset: step;
}
.stepper li {
  counter-increment: step;
  color: var(--ink-soft); font-size: 0.85rem; font-weight: 600;
  padding: 0.3rem 0.7rem; border-radius: 999px; border: 1px solid var(--line);
  background: var(--surface);
}
.stepper li::before { content: counter(step) " · "; opacity: 0.6; }
.stepper li.active { color: var(--accent-ink); background: var(--accent); border-color: var(--accent); }
.stepper li.done { color: var(--good); border-color: var(--good); }

main { padding: 1.5rem 0 3rem; }

.card {
  background: var(--surface); border: 1px solid var(--line);
  border-radius: var(--radius); box-shadow: var(--shadow);
  padding: 1.5rem; margin-bottom: 1.25rem;
}
.card h2 { margin: 0 0 0.25rem; font-size: 1.25rem; letter-spacing: -0.01em; }
.card .sub { color: var(--ink-soft); margin: 0 0 1rem; font-size: 0.95rem; }

label { display: block; font-weight: 600; margin: 1rem 0 0.35rem; }
label .opt { font-weight: 400; color: var(--ink-soft); font-size: 0.85rem; }
textarea, input[type="text"] {
  width: 100%; padding: 0.65rem 0.8rem;
  border: 1px solid var(--line); border-radius: 8px;
  background: var(--bg); color: var(--ink);
  font: inherit;
}
textarea { min-height: 7.5rem; resize: vertical; }
textarea:focus, input:focus { outline: 2px solid var(--accent); outline-offset: -1px; }

button {
  font: inherit; font-weight: 600; cursor: pointer;
  border: 1px solid transparent; border-radius: 8px;
  padding: 0.6rem 1.2rem;
}
button.primary { background: var(--accent); color: var(--accent-ink); }
button.primary:hover { filter: brightness(1.08); }
button.ghost { background: transparent; color: var(--accent); border-color: var(--accent); }
button:disabled { opacity: 0.5; cursor: not-allowed; }
.actions { margin-top: 1.5rem; display: flex; gap: 0.75rem; flex-wrap: wrap; }

.question { border-top: 1px solid var(--line); padding: 1.1rem 0; }
.question:first-of-type { border-top: none; }
.question .qnum { font-size: 0.8rem; font-weight: 700; color: var(--accent); text-transform: uppercase; letter-spacing: 0.05em; }
.question .prompt { margin: 0.3rem 0 0.7rem; }
.choice {
  display: flex; gap: 0.6rem; align-items: baseline;
  padding: 0.45rem 0.6rem; border-radius: 8px; margin: 0.15rem 0;
  cursor: pointer; font-weight: 400;
}
.choice:hover { background: var(--bg); }
.choice input { accent-color: var(--accent); }
.choice .label { font-weight: 700; min-width: 1.2rem; }

.badge {
  display: inline-block; padding: 0.15rem 0.6rem; border-radius: 999px;
  font-size: 0.8rem; font-weight: 700;
}
.badge.solid { color: var(--good); background: var(--good-bg); }
.badge.shaky { color: var(--warn); background: var(--warn-bg); }
.badge.missing { color: var(--bad); background: var(--bad-bg); }
.skill-row { display: flex; gap: 0.75rem; align-items: baseline; padding: 0.4rem 0; }
.skill-row .evidence { color: var(--ink-soft); font-size: 0.9rem; }

.worked {
  background: var(--bg); border: 1px solid var(--line);
  border-radius: 8px; padding: 1rem 1.25rem; margin: 0.75rem 0;
}
.worked ol { margin: 0.5rem 0 0; padding-left: 1.4rem; }
.worked ol li { margin: 0.4rem 0; }

.reveal { margin-top: 0.6rem; }
.reveal .answer { font-weight: 700; }
.selfmark { display: flex; gap: 0.5rem; margin-top: 0.5rem; align-items: center; font-size: 0.9rem; }
.selfmark button { padding: 0.3rem 0.8rem; font-size: 0.85rem; }
.selfmark button.picked-yes { background: var(--good-bg); color: var(--good); border-color: var(--good); }
.selfmark button.picked-no { background: var(--bad-bg); color: var(--bad); border-color: var(--bad); }

.result { border-left: 4px solid var(--line); padding: 0.6rem 1rem; margin: 0.75rem 0; border-radius: 0 8px 8px 0; }
.result.correct { border-color: var(--good); background: var(--good-bg); }
.result.incorrect { border-color: var(--bad); background: var(--bad-bg); }
.result .verdict { font-weight: 700; }
.result details { margin-top: 0.4rem; }
.result summary { cursor: pointer; color: var(--ink-soft); font-size: 0.9rem; }

.loading { display: flex; align-items: center; gap: 0.9rem; padding: 2.5rem 1rem; justify-content: center; color: var(--ink-soft); }
.spinner {
  width: 22px; height: 22px; border-radius: 50%;
  border: 3px solid var(--line); border-top-color: var(--accent);
  animation: spin 0.8s linear infinite; flex: none;
}
@keyframes spin { to { transform: rotate(360deg); } }

.error-banner {
  background: var(--bad-bg); color: var(--bad);
  border: 1px solid var(--bad); border-radius: var(--radius);
  padding: 1rem 1.25rem; margin-bottom: 1.25rem;
}
.error-banner .msg { white-space: pre-wrap; word-break: break-word; font-size: 0.9rem; }
.error-banner button { margin-top: 0.6rem; }

.prose h1, .prose h2, .prose h3 { letter-spacing: -0.01em; margin-top: 1.6em; }
.prose h1:first-child, .prose h2:first-child { margin-top: 0; }
.prose pre { background: var(--bg); padding: 0.8rem 1rem; border-radius: 8px; overflow-x: auto; }
.prose blockquote { border-left: 3px solid var(--accent); margin-left: 0; padding-left: 1rem; color: var(--ink-soft); }
.prose table { border-collapse: collapse; }
.prose td, .prose th { border: 1px solid var(--line); padding: 0.4rem 0.7rem; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: 0.2rem 0; }

.section-nav { display: flex; gap: 0.4rem; padding-top: 1rem; }
.section-nav button {
  background: transparent; color: var(--ink-soft);
  border: 1px solid var(--line); border-radius: 999px;
  padding: 0.35rem 0.9rem; font-size: 0.9rem;
}
.section-nav button.active { background: var(--ink); color: var(--surface); border-color: var(--ink); }

.qhead { display: flex; justify-content: space-between; align-items: center; gap: 0.75rem; }
button.star {
  background: transparent; color: var(--ink-soft);
  border: 1px solid var(--line); border-radius: 999px;
  padding: 0.2rem 0.75rem; font-size: 0.85rem;
}
button.star.on { color: #b8860b; border-color: #b8860b; background: rgba(218, 165, 32, 0.12); }
button.music {
  background: transparent; color: var(--ink-soft);
  border: 1px solid var(--line); border-radius: 999px;
  padding: 0.2rem 0.75rem; font-size: 0.85rem; flex: none;
}
button.music.on { color: var(--accent); border-color: var(--accent); background: rgba(116, 143, 252, 0.12); }

.lib-head { display: flex; justify-content: space-between; align-items: baseline; gap: 0.75rem; margin-bottom: 0.6rem; }
.lib-meta { color: var(--ink-soft); font-size: 0.85rem; }
.lib-item details { margin-top: 0.6rem; }
.lib-item summary { cursor: pointer; color: var(--ink-soft); font-size: 0.9rem; }

.chat-thread { display: flex; flex-direction: column; gap: 0.6rem; margin: 1rem 0; }
.msg { max-width: 85%; padding: 0.6rem 0.95rem; border-radius: 14px; }
.msg :first-child { margin-top: 0; }
.msg :last-child { margin-bottom: 0; }
.msg.coach { background: var(--bg); border: 1px solid var(--line); align-self: flex-start; border-bottom-left-radius: 4px; }
.msg.student { background: var(--accent); color: var(--accent-ink); align-self: flex-end; border-bottom-right-radius: 4px; white-space: pre-wrap; }
.msg .spinner { margin: 0.1rem 0; }
.chat-input-row { display: flex; gap: 0.6rem; align-items: flex-end; }
.chat-input-row textarea { flex: 1; min-height: 2.8rem; }

.attach-row { display: flex; align-items: center; gap: 0.55rem; margin-top: 0.45rem; color: var(--ink-soft); font-size: 0.85rem; }
button.attach-icon {
  display: inline-flex; align-items: center; justify-content: center;
  width: 2rem; height: 2rem; padding: 0; flex: none;
  border: 1px solid var(--line); border-radius: 8px;
  background: transparent; color: var(--ink-soft);
}
button.attach-icon:hover { color: var(--accent); border-color: var(--accent); }
textarea.drop-target {
  outline: 2px dashed var(--accent); outline-offset: -2px;
  background: color-mix(in srgb, var(--accent) 7%, var(--bg));
}
.attach-error { color: var(--bad); font-size: 0.9rem; margin-top: 0.4rem; min-height: 1.2em; }
.img-preview { display: flex; align-items: flex-start; gap: 0.75rem; margin-top: 0.5rem; }
.img-preview img {
  max-height: 180px; max-width: 100%;
  border: 1px solid var(--line); border-radius: 8px;
}

.site-footer { color: var(--ink-soft); font-size: 0.8rem; padding-bottom: 2rem; }