:root {
  --bg: #0f172a;
  --panel: #ffffff;
  --ink: #1e293b;
  --muted: #64748b;
  --brand: #4f46e5;
  --brand-dark: #4338ca;
  --accent: #0ea5e9;
  --ok: #16a34a;
  --ok-bg: #dcfce7;
  --bad: #dc2626;
  --bad-bg: #fee2e2;
  --line: #e2e8f0;
  --soft: #f8fafc;
}

* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  color: var(--ink);
  background: var(--soft);
  line-height: 1.55;
}

/* Header */
.topbar {
  background: linear-gradient(120deg, var(--brand), var(--accent));
  color: #fff;
  padding: 22px 26px;
}
.badge {
  display: inline-block;
  background: rgba(255,255,255,.2);
  padding: 3px 10px;
  border-radius: 999px;
  font-size: 12px;
  font-weight: 600;
  letter-spacing: .3px;
}
.topbar h1 { margin: 8px 0 2px; font-size: 26px; }
.topbar p { margin: 0; opacity: .9; font-size: 14px; }

/* Layout */
.layout {
  display: grid;
  grid-template-columns: 300px 1fr;
  gap: 22px;
  max-width: 1100px;
  margin: 24px auto;
  padding: 0 20px;
}
@media (max-width: 820px) {
  .layout { grid-template-columns: 1fr; }
}

/* Sidebar */
.sidebar {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 18px;
  height: fit-content;
}
.sidebar h2 { margin: 0 0 4px; font-size: 16px; }
.hint { color: var(--muted); font-size: 13px; margin: 0 0 12px; }
.topics { list-style: none; margin: 0; padding: 0; }
.topics li {
  padding: 10px 12px;
  border-radius: 10px;
  cursor: pointer;
  border: 1px solid transparent;
  transition: .15s;
}
.topics li:hover { background: var(--soft); border-color: var(--line); }
.topics li.active { background: #eef2ff; border-color: #c7d2fe; }
.topics .t-title { font-weight: 600; font-size: 14px; }
.topics .t-desc { color: var(--muted); font-size: 12px; }

/* Content */
.content { min-width: 0; }
.ask { display: flex; gap: 10px; margin-bottom: 16px; }
.ask input {
  flex: 1;
  padding: 13px 15px;
  border: 1px solid var(--line);
  border-radius: 12px;
  font-size: 15px;
  outline: none;
}
.ask input:focus { border-color: var(--brand); box-shadow: 0 0 0 3px #e0e7ff; }
.ask button {
  padding: 0 22px;
  background: var(--brand);
  color: #fff;
  border: 0;
  border-radius: 12px;
  font-weight: 600;
  font-size: 15px;
  cursor: pointer;
}
.ask button:hover { background: var(--brand-dark); }
.ask button:disabled { opacity: .5; cursor: default; }

.status {
  background: #eff6ff;
  color: #1d4ed8;
  border: 1px solid #bfdbfe;
  padding: 12px 14px;
  border-radius: 10px;
  margin-bottom: 14px;
  font-size: 14px;
}
.status.error { background: var(--bad-bg); color: var(--bad); border-color: #fecaca; }

/* Welcome card */
.welcome {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 22px;
}
.welcome h2 { margin-top: 0; }

/* Lesson blocks */
.lesson { display: grid; gap: 16px; }
.card {
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 18px 20px;
}
.card h3 {
  margin: 0 0 10px;
  font-size: 16px;
  display: flex; align-items: center; gap: 8px;
}
.card h3 .ic { font-size: 18px; }
.lesson p { margin: 8px 0; }
.examples { margin: 0; padding-left: 18px; }
.examples li { margin: 8px 0; }

/* MCQ */
.mcq { border-top: 1px dashed var(--line); padding-top: 14px; margin-top: 6px; }
.mcq:first-of-type { border-top: 0; padding-top: 0; }
.mcq .q { font-weight: 600; margin-bottom: 8px; }
.opts { display: grid; gap: 8px; }
.opt {
  display: flex; align-items: center; gap: 10px;
  padding: 10px 12px;
  border: 1px solid var(--line);
  border-radius: 10px;
  cursor: pointer;
}
.opt:hover { background: var(--soft); }
.opt input { accent-color: var(--brand); }
.opt.correct { background: var(--ok-bg); border-color: #86efac; }
.opt.wrong { background: var(--bad-bg); border-color: #fca5a5; }
.opt.disabled { cursor: default; }
.submit-mcq {
  margin-top: 10px;
  padding: 8px 16px;
  background: var(--accent);
  color: #fff; border: 0; border-radius: 9px; font-weight: 600; cursor: pointer;
}
.submit-mcq:disabled { opacity: .5; cursor: default; }
.mcq-explain {
  margin-top: 10px;
  background: var(--soft);
  border-left: 3px solid var(--brand);
  padding: 10px 12px;
  border-radius: 6px;
  font-size: 14px;
}
.mcq-explain.ok { border-color: var(--ok); }
.mcq-explain.no { border-color: var(--bad); }

/* Next topic */
.next-topic { display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap; }
.next-topic button {
  padding: 10px 18px; background: var(--brand); color: #fff; border: 0;
  border-radius: 10px; font-weight: 600; cursor: pointer;
}
.sources { color: var(--muted); font-size: 12px; margin-top: 4px; }

.foot {
  text-align: center;
  color: var(--muted);
  font-size: 12px;
  padding: 18px;
}

/* spinner */
.spinner {
  display: inline-block; width: 14px; height: 14px;
  border: 2px solid #bfdbfe; border-top-color: #1d4ed8;
  border-radius: 50%; animation: spin .7s linear infinite; vertical-align: -2px; margin-right: 8px;
}
@keyframes spin { to { transform: rotate(360deg); } }