:root {
  --bg: #f4f7fb;
  --card: #ffffff;
  --ink: #15213a;
  --muted: #5a6780;
  --ok: #0f8a5f;
  --bad: #b42318;
  --accent: #0a66c2;
  --line: #d9e2f2;
}

* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: "Avenir Next", "Segoe UI", sans-serif;
  background: linear-gradient(180deg, #f7fbff 0%, var(--bg) 100%);
  color: var(--ink);
}
.app {
  max-width: 760px;
  margin: 0 auto;
  padding: 16px;
}
.top h1 { margin: 0 0 4px; font-size: 1.5rem; }
.sub { margin: 0 0 14px; color: var(--muted); font-size: .95rem; }
.card {
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: 16px;
  padding: 14px;
  box-shadow: 0 8px 20px rgba(16, 24, 40, 0.06);
  margin-bottom: 14px;
}
.meta {
  display: flex;
  justify-content: space-between;
  color: var(--muted);
  font-size: .9rem;
}
h2 { font-size: 1.1rem; margin: 10px 0 12px; line-height: 1.35; }
.options { display: grid; gap: 8px; }
.auth-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 6px;
}
.auth-grid input,
#verify-code-form input {
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 10px;
  font-size: .95rem;
}
.msg { margin: 10px 0 0; color: var(--muted); font-size: .92rem; }
.lesson-block h3 { margin: 0 0 8px; font-size: 1rem; }
.lesson-block p { margin: 6px 0; line-height: 1.4; }
.lesson-block ul { margin: 6px 0 0 18px; padding: 0; }
.lesson-block li { margin: 4px 0; }
.lesson-example {
  border: 1px solid var(--line);
  border-radius: 10px;
  background: #f8fbff;
  padding: 8px 10px;
  margin: 8px 0;
}
.option {
  border: 1px solid var(--line);
  border-radius: 12px;
  padding: 10px 12px;
  background: #fff;
  text-align: left;
  font-size: .95rem;
}
.option:disabled { opacity: 1; }
.option.selected { border-color: var(--accent); background: #edf5ff; }
.feedback {
  margin-top: 12px;
  border-radius: 12px;
  padding: 10px 12px;
  border: 1px solid;
  font-size: .95rem;
  line-height: 1.4;
}
.feedback.ok { border-color: #8fe3c6; background: #e8fff5; color: #0f5132; }
.feedback.bad { border-color: #f4b4ad; background: #fff1ef; color: #7a271a; }
.hidden { display: none; }
.actions { margin-top: 12px; display: flex; gap: 8px; flex-wrap: wrap; }
.btn {
  border: 0;
  border-radius: 10px;
  background: var(--accent);
  color: #fff;
  padding: 10px 14px;
  font-weight: 600;
}
.btn:disabled { opacity: .45; }
.btn.secondary { background: #1f7a5b; }
.btn.ghost { background: #5a6780; }
.config textarea {
  width: 100%;
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 10px;
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
}
code { background: #f3f4f6; padding: 1px 5px; border-radius: 4px; }
.teacher-stats { margin-top: 12px; }
.teacher-form {
  display: grid;
  grid-template-columns: 1fr;
  gap: 6px;
  margin: 10px 0 6px;
}
.teacher-form input {
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 10px;
  font-size: .95rem;
}
.checkline {
  display: flex;
  align-items: center;
  gap: 8px;
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 10px;
  background: #fff;
}
.teacher-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 10px;
}
.stat-box {
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 10px;
  background: #fafcff;
}
.stat-box h4 {
  margin: 0 0 8px;
  font-size: .95rem;
}
.stat-box p {
  margin: 4px 0;
  font-size: .92rem;
}
.table-wrap { overflow-x: auto; margin-top: 8px; }
table { width: 100%; border-collapse: collapse; font-size: .9rem; }
th, td { border-bottom: 1px solid var(--line); padding: 8px 6px; text-align: left; white-space: nowrap; }
th { background: #f8fbff; }
@media (min-width: 768px) {
  .top h1 { font-size: 1.8rem; }
  h2 { font-size: 1.2rem; }
  .teacher-grid { grid-template-columns: 1fr 1fr; }
  .teacher-form { grid-template-columns: 1fr 1fr; align-items: end; }
  .auth-grid { grid-template-columns: 1fr 1fr; }
}