/* Classroom LMS —— 全站样式（任务 9/10 扩展） */
:root {
  --fg: #1f2328;
  --muted: #6a737d;
  --line: #e1e4e8;
  --brand: #2563eb;
  --bg: #ffffff;
  --bg-soft: #f6f8fa;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", "Segoe UI", sans-serif;
  color: var(--fg);
  background: var(--bg);
  line-height: 1.6;
}
a { color: var(--brand); text-decoration: none; }
a:hover { text-decoration: underline; }
button, .btn {
  display: inline-block;
  padding: 8px 16px;
  border: 1px solid var(--brand);
  border-radius: 6px;
  background: var(--brand);
  color: #fff;
  font-size: 15px;
  cursor: pointer;
}
button.ghost, .btn.ghost { background: #fff; color: var(--brand); }
.container { max-width: 960px; margin: 0 auto; padding: 16px; }
.muted { color: var(--muted); }

/* ---- 管理后台布局（本文件仅 /admin 页面引用） ---- */
body { background: var(--bg-soft); }
header { background: #fff; border-bottom: 1px solid var(--line); padding: 12px 24px; display: flex; align-items: center; gap: 28px; flex-wrap: wrap; position: sticky; top: 0; z-index: 10; }
header h1 { font-size: 19px; margin: 0; white-space: nowrap; }
header nav { display: flex; gap: 4px; flex-wrap: wrap; }
header nav a { padding: 6px 12px; border-radius: 6px; color: var(--fg); font-size: 14.5px; }
header nav a:hover { background: var(--bg-soft); text-decoration: none; }
header nav a.active { background: var(--brand); color: #fff; }
main { max-width: 1080px; margin: 24px auto 0; padding: 0 24px 64px; }
section { background: #fff; border: 1px solid var(--line); border-radius: 10px; padding: 20px 24px 24px; margin-bottom: 24px; }
section.narrow { max-width: 460px; margin: 48px auto 24px; }
h2 { font-size: 18px; margin: 0 0 14px; }
h3 { font-size: 15.5px; margin: 22px 0 10px; }
h2 small, h3 small { color: var(--muted); font-weight: 400; }
table { border-collapse: collapse; width: 100%; margin: 12px 0; }
th, td { border: 1px solid var(--line); padding: 8px 10px; text-align: left; font-size: 14px; vertical-align: middle; }
th { background: var(--bg-soft); font-weight: 600; white-space: nowrap; }
tr:hover td { background: #fafbfc; }
input, select, textarea { font: inherit; padding: 7px 10px; border: 1px solid var(--line); border-radius: 6px; background: #fff; max-width: 100%; }
input:focus, select:focus, textarea:focus { outline: 2px solid rgba(37, 99, 235, 0.25); border-color: var(--brand); }
textarea { width: 100%; }
button { font: inherit; }
button:disabled { opacity: 0.45; cursor: not-allowed; }
button.ghost { border-color: var(--line); }
form.inline { display: inline-flex; gap: 8px; align-items: center; vertical-align: middle; }
form.inline input[type='text'], form.inline input:not([type]) { width: 200px; }
label.block { display: block; margin: 6px 0; }
.error { color: #b91c1c; }
p.error { background: #fef2f2; border: 1px solid #fecaca; padding: 8px 12px; border-radius: 6px; }
.markdown-preview { border: 1px dashed var(--line); border-radius: 8px; padding: 12px 16px; margin: 8px 0; background: var(--bg-soft); }
code { background: var(--bg-soft); padding: 2px 6px; border-radius: 4px; font-size: 13px; word-break: break-all; }
form p { margin: 12px 0; }
form p label { display: inline-flex; align-items: center; gap: 8px; flex-wrap: wrap; }
form p label:has(textarea) { display: block; }
span[data-upload-status] { color: var(--muted); font-size: 13.5px; }