* { box-sizing: border-box; margin: 0; padding: 0; }
:root {
  --bg: #f3f4fb; --card: #ffffff; --ink: #1f2937; --muted: #6b7280;
  --primary: #6366f1; --primary-dark: #4f46e5;
  --violet: #8b5cf6; --pink: #ec4899; --cyan: #06b6d4;
  --green: #16a34a; --red: #dc2626; --amber: #d97706; --border: #e8eaf1; --radius: 18px;
  --grad: linear-gradient(135deg,#6366f1,#8b5cf6 55%,#ec4899);
}
body { font-family: "Segoe UI", "PingFang SC", "Microsoft YaHei", system-ui, sans-serif; background: var(--bg); color: var(--ink); min-height: 100vh; display: flex; flex-direction: column; line-height: 1.55; }

/* header */
header { background: rgba(255,255,255,.85); backdrop-filter: blur(10px); color: var(--ink); padding: 12px 24px; display: flex; align-items: center; justify-content: space-between; flex-wrap: wrap; gap: 10px; position: sticky; top: 0; z-index: 10; box-shadow: 0 2px 14px rgba(15,23,42,.06); border-bottom: 1px solid var(--border); }
.brand { font-size: 1.15rem; font-weight: 800; background: var(--grad); -webkit-background-clip: text; background-clip: text; color: transparent; }
nav { display: flex; gap: 4px; flex-wrap: wrap; }
.nav-btn { background: transparent; color: var(--muted); border: none; padding: 7px 13px; border-radius: 12px; cursor: pointer; font-size: .9rem; font-weight: 600; transition: .15s; }
.nav-btn:hover { background: #eef0fb; color: var(--primary); }
.nav-btn.active { background: var(--grad); color: #fff; box-shadow: 0 4px 14px rgba(99,102,241,.35); }
main { flex: 1; width: 100%; max-width: 1120px; margin: 0 auto; padding: 22px 20px 44px; }
.view { display: none; animation: fadeIn .25s ease; }
.view.active { display: block; }
@keyframes fadeIn { from { opacity: 0; transform: translateY(5px); } to { opacity: 1; transform: none; } }
h2 { margin-bottom: 16px; font-size: 1.3rem; }
h3 { margin-bottom: 10px; }

/* buttons + forms */
button { padding: 9px 18px; border: 1px solid var(--border); background: #fff; border-radius: 12px; cursor: pointer; font-size: .92rem; transition: background .15s, transform .1s, box-shadow .15s; }
button:hover { background: #f6f7fb; }
button:active { transform: scale(.98); }
button.primary { background: var(--grad); border-color: transparent; color: #fff; font-weight: 700; box-shadow: 0 6px 16px rgba(99,102,241,.3); }
button.primary:hover { filter: brightness(1.05); }
button.danger { background: #fff; border-color: var(--red); color: var(--red); }
select, input[type="text"], input[type="password"], input[type="number"] { padding: 9px 12px; border: 1px solid var(--border); border-radius: 11px; font-size: .92rem; background: #fff; color: var(--ink); min-width: 180px; }
input[type="text"], input[type="password"], input[type="number"] { flex: 1; min-width: 240px; }
select:focus, input:focus { outline: 2px solid var(--primary); outline-offset: 1px; }

.card { background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; margin-bottom: 16px; box-shadow: 0 6px 16px rgba(15,23,42,.04); }
.toolbar { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; margin-bottom: 16px; }

/* home hero */
.ai-hero { display: flex; align-items: center; gap: 16px; background: var(--grad); color: #fff; border-radius: var(--radius); padding: 20px 22px; margin-bottom: 20px; cursor: pointer; box-shadow: 0 18px 40px rgba(99,102,241,.35); transition: transform .15s, box-shadow .15s; position: relative; overflow: hidden; }
.ai-hero::after { content: "✨"; position: absolute; right: 22px; top: 14px; font-size: 2.4rem; opacity: .5; transform: rotate(15deg); }
.ai-hero:hover { transform: translateY(-2px); box-shadow: 0 22px 48px rgba(79,70,229,.42); }
.ai-hero-icon { font-size: 2.3rem; line-height: 1; }
.ai-hero-body { flex: 1; min-width: 0; }
.ai-hero-title { font-size: 1.25rem; font-weight: 800; }
.ai-hero-sub { font-size: .87rem; opacity: .92; margin-top: 4px; }
.ai-hero-badge { background: rgba(255,255,255,.18); border: 1px solid rgba(255,255,255,.35); color: #fff; font-weight: 800; font-size: .78rem; padding: 5px 12px; border-radius: 999px; white-space: nowrap; }

/* course grid */
.course-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 16px; }
.course-card { background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 18px; cursor: pointer; transition: .18s; position: relative; box-shadow: 0 6px 16px rgba(15,23,42,.04); }
.course-card:hover { transform: translateY(-4px); box-shadow: 0 16px 34px rgba(79,70,229,.16); border-color: #c7d2fe; }
.course-card .c-code { position: absolute; top: 14px; right: 14px; font-size: .7rem; font-weight: 700; padding: 3px 9px; border-radius: 999px; }
.course-card .c-code.doing { background: #dcfce7; color: #16a34a; }
.course-card .c-code.todo { background: #fee2e2; color: #dc2626; }
.course-card .c-icon { width: 46px; height: 46px; border-radius: 13px; display: flex; align-items: center; justify-content: center; font-size: 1.4rem; margin-bottom: 12px; color: #fff; }
.course-card .c-name { font-size: 1.02rem; font-weight: 800; }
.course-card .c-meta { color: var(--muted); font-size: .8rem; margin-top: 8px; }
.course-card .c-tag { display: inline-block; background: #eef2ff; color: var(--primary-dark); border-radius: 8px; padding: 2px 9px; font-size: .76rem; margin-top: 4px; }
.course-card .c-progress { color: var(--muted); font-size: .78rem; margin-top: 10px; }
.course-card .c-progress .bar { height: 7px; border-radius: 999px; background: #eef0f7; margin-top: 5px; overflow: hidden; }
.course-card .c-progress .fill { height: 100%; border-radius: 999px; }
.stats-bar { margin-top: 20px; color: var(--muted); font-size: .9rem; }

/* progress */
.progress-bar { background: #eef0f7; border: 1px solid var(--border); border-radius: 999px; height: 10px; overflow: hidden; margin-top: 6px; max-width: 340px; }
.progress-fill { background: var(--grad); height: 100%; width: 0; transition: width .3s; }

/* question / quiz */
.q-card { background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 20px; margin-bottom: 14px; box-shadow: 0 6px 16px rgba(15,23,42,.04); }
.q-head { display: flex; justify-content: space-between; align-items: center; margin-bottom: 10px; gap: 8px; flex-wrap: wrap; }
.q-badge { background: #eef2ff; color: var(--primary-dark); border-radius: 999px; padding: 3px 11px; font-size: .78rem; font-weight: 700; }
.q-text { font-size: 1rem; margin-bottom: 14px; line-height: 1.6; }
.opt { display: block; width: 100%; text-align: left; background: #fff; border: 1.5px solid var(--border); border-radius: 12px; padding: 11px 14px; margin-bottom: 8px; font-size: .92rem; cursor: pointer; transition: .12s; }
.opt:hover { border-color: var(--primary); background: #f5f6ff; }
.opt.selected { border-color: var(--primary); background: #eef2ff; font-weight: 700; }
.opt.correct { border-color: var(--green); background: #f0fdf4; font-weight: 700; }
.opt.wrong { border-color: var(--red); background: #fef2f2; }
.opt.dim { opacity: .45; }
.feedback { margin-top: 12px; padding: 12px 14px; border-radius: 12px; font-size: .92rem; line-height: 1.6; display: none; }
.feedback.ok { background: #f0fdf4; border: 1px solid #bbf7d0; color: #14532d; display: block; }
.feedback.bad { background: #fef2f2; border: 1px solid #fecaca; color: #7f1d1d; display: block; }
.feedback .exp { margin-top: 6px; color: var(--muted); }
.score-banner { background: var(--grad); color: #fff; border-radius: var(--radius); padding: 24px; text-align: center; margin-bottom: 18px; box-shadow: 0 14px 30px rgba(99,102,241,.3); }
.score-banner .big { font-size: 2.2rem; font-weight: 800; }
.score-banner .sub { opacity: .92; margin-top: 4px; }

/* chat (AI) */
.ask-box { background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 16px; margin-bottom: 16px; box-shadow: 0 6px 16px rgba(15,23,42,.04); }
.msg-user { background: #eef2ff; border: 1px solid #c7d2fe; border-radius: 14px; padding: 11px 14px; margin: 8px 0; }
.msg-assistant { background: linear-gradient(180deg,#f5f3ff,#fdf4ff); border: 1px solid #ddd6fe; border-radius: 14px; padding: 13px 16px; margin: 8px 0; }
.msg-role { font-size: .78rem; color: var(--muted); margin-bottom: 5px; }
.ask-block { margin-bottom: 12px; }

/* source / related */
.source-list, .rel-topics { margin-top: 12px; }
.source-item { background: #f9fafb; border: 1px solid var(--border); border-radius: 12px; padding: 10px 12px; margin-top: 8px; font-size: .88rem; }
.source-item .src-head { display: flex; justify-content: space-between; gap: 8px; flex-wrap: wrap; }
.source-item .src-file { font-weight: 700; color: var(--primary-dark); }
.source-item .src-score { color: var(--muted); font-size: .8rem; }
.source-item .src-snip { color: var(--muted); margin-top: 6px; line-height: 1.55; }
.rel-topics .rel-label { color: var(--muted); font-size: .8rem; margin-bottom: 6px; }
.rel-topics .rel-chips { display: flex; flex-wrap: wrap; gap: 8px; }
.rel-chip { background: #eef2ff; color: var(--primary-dark); border: 1px solid #c7d2fe; border-radius: 999px; padding: 6px 14px; font-size: .85rem; cursor: pointer; }
.rel-chip:hover { background: #dbeafe; }

/* mistake */
.mistake-item { background: var(--card); border: 1px solid var(--border); border-left: 4px solid var(--amber); border-radius: 12px; padding: 14px 16px; margin-bottom: 10px; }
.mistake-item .m-q { font-weight: 600; margin-bottom: 6px; }
.mistake-item .m-meta { color: var(--muted); font-size: .84rem; margin-bottom: 6px; }
.mistake-item .m-wrong { color: var(--red); }
.mistake-item .m-right { color: var(--green); font-weight: 600; }
.mistake-item .m-expl { margin-top: 6px; color: var(--muted); line-height: 1.6; }
.mastery-pill { display: inline-block; font-size: .76rem; font-weight: 600; border-radius: 999px; padding: 2px 10px; margin-left: 6px; }
.mastery-new { background: #fee2e2; color: #b91c1c; }
.mastery-progress { background: #fef3c7; color: #b45309; }
.mastery-done { background: #dcfce7; color: #15803d; }

/* notes */
.note-btn { background: #fef3c7; color: #b45309; border: 1px solid #fde68a; border-radius: 999px; padding: 5px 12px; font-size: .82rem; cursor: pointer; }
.note-btn:hover { background: #fde68a; }
.note-item { background: var(--card); border: 1px solid var(--border); border-radius: 12px; padding: 14px 16px; margin-bottom: 10px; }
.note-item .note-text { color: var(--ink); line-height: 1.7; margin-top: 4px; }
#note-toggle-star.active { background: #fef3c7; border-color: #f59e0b; color: #78350f; font-weight: 700; }
#note-modal textarea { min-height: 120px; resize: vertical; font-family: inherit; width: 100%; padding: 10px 12px; border: 1px solid var(--border); border-radius: 11px; }

/* rich text */
.rich-p { margin: 0 0 .6em; line-height: 1.85; }
.rich-h1 { font-size: 1.1rem; font-weight: 800; color: var(--primary-dark); margin: .7em 0 .35em; padding-left: 8px; border-left: 4px solid var(--primary); }
.rich-h2 { font-size: 1rem; font-weight: 700; color: var(--primary-dark); margin: .7em 0 .3em; }
.rich-h3, .rich-h4 { font-size: .96rem; font-weight: 700; color: var(--ink); margin: .6em 0 .25em; }
.rich-list { margin: .35em 0 .7em; padding-left: 22px; }
.rich-list li { margin-bottom: .4em; line-height: 1.75; }
.rich-quote { border-left: 3px solid var(--green); background: #f0fdf4; padding: 8px 12px; border-radius: 0 8px 8px 0; color: #14532d; margin: .4em 0; line-height: 1.7; }
.rich-table { width: 100%; border-collapse: collapse; margin: .5em 0 .8em; font-size: .88rem; }
.rich-table th, .rich-table td { border: 1px solid var(--border); padding: 6px 10px; text-align: left; }
.rich-table th { background: #f0f6ff; font-weight: 700; color: var(--primary-dark); }

/* lesson */
.lesson-toc { display: flex; gap: 8px; flex-wrap: wrap; margin-bottom: 16px; }
.lesson-toc button { background: #eef2ff; color: var(--primary-dark); border: 1px solid #c7d2fe; border-radius: 999px; padding: 6px 14px; font-size: .85rem; }
.lesson-toc button:hover { background: #dbeafe; }
.lesson-toc button.active { background: var(--grad); color: #fff; border-color: transparent; }
.lesson-card { background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 22px; margin-bottom: 18px; box-shadow: 0 6px 16px rgba(15,23,42,.04); }
.lesson-card h3 { font-size: 1.12rem; margin-bottom: 12px; color: var(--primary-dark); }
.lesson-summary { line-height: 1.85; margin-bottom: 12px; }
.lesson-summary b, .lesson-points b { color: var(--primary-dark); }
.lesson-points { margin: 10px 0 16px 4px; padding-left: 18px; line-height: 1.8; }
.lesson-points li { margin-bottom: 6px; }
.lesson-example { background: #f8fafc; border: 1px solid var(--border); border-left: 4px solid var(--primary); border-radius: 12px; padding: 14px 16px; margin-top: 12px; }
.lesson-example .ex-title { font-weight: 700; margin-bottom: 8px; }
.lesson-example .ex-q { line-height: 1.7; margin-bottom: 8px; }
.lesson-example .ex-steps { margin: 8px 0 8px 4px; padding-left: 18px; line-height: 1.75; color: var(--muted); font-size: .92rem; }
.lesson-example .ex-answer { background: #f0fdf4; border: 1px solid #bbf7d0; border-radius: 10px; padding: 8px 12px; color: #14532d; font-weight: 600; margin-top: 8px; }
.lesson-example .ex-src { color: var(--muted); font-size: .8rem; margin-top: 8px; }
.lesson-more { margin-top: 18px; padding: 14px; background: #fffbeb; border: 1px solid #fde68a; border-radius: 12px; font-size: .92rem; line-height: 1.7; color: #78350f; }
.subtopic { border: 1px solid var(--border); border-radius: 14px; margin-bottom: 12px; background: #fff; overflow: hidden; }
.subtopic > summary { cursor: pointer; padding: 13px 16px; font-weight: 700; font-size: 1rem; background: #f8fafc; color: var(--primary-dark); list-style: none; user-select: none; }
.subtopic > summary::-webkit-details-marker { display: none; }
.subtopic > summary::before { content: '▶ '; font-size: .8rem; color: var(--primary); }
.subtopic[open] > summary::before { content: '▼ '; }
.subtopic > summary:hover { background: #eef2ff; }
.subtopic-body { padding: 14px 16px 16px; }
.quiz-drawer { margin-top: 14px; border: 1px solid #c7d2fe; border-radius: 12px; background: #fafaff; }
.quiz-drawer > summary { cursor: pointer; padding: 10px 14px; font-weight: 600; color: var(--primary-dark); list-style: none; border-radius: 12px; }
.quiz-drawer > summary::-webkit-details-marker { display: none; }
.quiz-drawer > summary::before { content: '▶ '; font-size: .8rem; }
.quiz-drawer[open] > summary::before { content: '▼ '; }
.quiz-mini { background: #fafaff; border: 1px dashed #c7d2fe; border-radius: 12px; padding: 12px 14px; margin: 12px 14px 4px; }
.quiz-mini .quiz-q { font-weight: 600; margin-bottom: 10px; line-height: 1.6; }
.quiz-fb { margin-top: 8px; padding: 8px 12px; border-radius: 10px; font-size: .9rem; line-height: 1.6; }
.quiz-fb.ok { background: #f0fdf4; border: 1px solid #bbf7d0; color: #14532d; }
.quiz-fb.bad { background: #fef2f2; border: 1px solid #fecaca; color: #7f1d1d; }
.src-file-btn { background: #eef2ff; color: var(--primary-dark); border: 1px solid #c7d2fe; border-radius: 999px; padding: 5px 12px; font-size: .82rem; max-width: 260px; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
.src-file-btn:hover { background: #dbeafe; }
.src-file-btn.active { background: var(--grad); color: #fff; border-color: transparent; }

/* auth */
.auth-box { max-width: 380px; margin: 60px auto 0; background: var(--card); border: 1px solid var(--border); border-radius: var(--radius); padding: 30px 28px; text-align: center; box-shadow: 0 12px 30px rgba(15,23,42,.08); }
.auth-box h2 { margin-bottom: 4px; }
.auth-box label { text-align: left; }
.auth-box input { width: 100%; min-width: 0; margin-bottom: 4px; }
.prog-btn { display: flex; flex-direction: column; align-items: flex-start; gap: 4px; width: 100%; padding: 16px 18px; margin-bottom: 12px; text-align: left; border-radius: 14px; background: #fff; border: 1px solid var(--border); cursor: pointer; }
.prog-btn:hover { border-color: var(--primary); background: #f5f6ff; }
.prog-btn .prog-ico { font-size: 1.5rem; }
.prog-btn .prog-name { font-size: 1.05rem; font-weight: 700; }
.prog-btn .prog-desc { font-size: .82rem; color: var(--muted); }
.prog-badge { display: inline-block; padding: 6px 14px; margin: 0 0 14px; border-radius: 999px; background: #eef2ff; border: 1px solid #c7d2fe; color: #4f46e5; font-weight: 700; font-size: .9rem; }

/* overlays / modals */
.guard-overlay { position: fixed; inset: 0; background: rgba(15,23,42,.55); display: flex; align-items: center; justify-content: center; z-index: 100; }
.guard-box { background: #fff; border-radius: 16px; padding: 30px; width: 340px; text-align: center; box-shadow: 0 20px 60px rgba(0,0,0,.35); }

/* toast + backtop + trend */
#toast { position: fixed; bottom: 76px; left: 50%; transform: translateX(-50%) translateY(20px); background: rgba(17,24,39,.92); color: #fff; padding: 10px 20px; border-radius: 999px; font-size: .9rem; z-index: 120; opacity: 0; pointer-events: none; transition: opacity .2s, transform .2s; box-shadow: 0 8px 24px rgba(0,0,0,.3); max-width: 90vw; text-align: center; }
#toast.show { opacity: 1; transform: translateX(-50%) translateY(0); }
#back-top { position: fixed; bottom: 22px; right: 22px; width: 44px; height: 44px; border-radius: 50%; background: var(--grad); color: #fff; border: none; font-size: 1.2rem; box-shadow: 0 6px 18px rgba(99,102,241,.4); cursor: pointer; z-index: 60; display: none; align-items: center; justify-content: center; }
#back-top:hover { transform: translateY(-2px); }
html { scroll-behavior: smooth; }
.trend-wrap { margin-top: 8px; display: flex; align-items: flex-end; gap: 6px; height: 90px; }
.trend-bar { flex: 1; display: flex; flex-direction: column; align-items: center; justify-content: flex-end; height: 100%; }
.trend-bar .tb { width: 100%; max-width: 34px; background: var(--grad); border-radius: 4px 4px 0 0; min-height: 2px; }
.trend-bar .tl { font-size: .68rem; color: var(--muted); margin-top: 4px; white-space: nowrap; }
.trend-bar .tpct { font-size: .66rem; color: var(--primary-dark); font-weight: 700; margin-bottom: 2px; }

/* misc */
.muted { color: var(--muted); }
.small { font-size: .85rem; }
label { display: block; font-size: .88rem; font-weight: 600; margin: 12px 0 4px; color: var(--ink); }
.empty { color: var(--muted); text-align: center; padding: 40px 0; }
footer { text-align: center; color: var(--muted); font-size: .82rem; padding: 16px; border-top: 1px solid var(--border); background: #fff; }
.loading { color: var(--muted); padding: 20px 0; }
.spin { display: inline-block; width: 14px; height: 14px; border: 2px solid var(--border); border-top-color: var(--primary); border-radius: 50%; animation: spin .7s linear infinite; vertical-align: -2px; margin-right: 6px; }
@keyframes spin { to { transform: rotate(360deg); } }
.ai-actions { display: flex; gap: 8px; flex-wrap: wrap; margin-top: 12px; padding-top: 10px; border-top: 1px dashed var(--border); }
.ai-actions button { background: #f8fafc; color: var(--primary-dark); border: 1px solid var(--border); font-size: .82rem; padding: 5px 12px; border-radius: 999px; }
.ai-actions button:hover { background: #eef2ff; }
/* 拍照提问：用户气泡里的缩略图、识别结果提示框、追问卡片 */
.ask-thumb { display: block; max-width: min(320px, 78vw); max-height: 260px; border-radius: 10px; margin: 8px 0 2px; border: 1px solid var(--border); background: #fff; }
.ask-followup { border-left: 3px solid var(--primary); padding-left: 12px; margin-left: 4px; }
.ask-drop { outline: 2px dashed var(--primary); outline-offset: 4px; background: #f0f7ff; border-radius: 10px; transition: background .15s; }
/* 输入框里的图片附件：默认**不显示**，用 .show 控制。
   ⚠️ 不能只靠 [hidden]：作者样式里的 display:flex 会覆盖 UA 的 [hidden]{display:none}，
   结果没图时也会露出一个没有 src 的破图框（已踩过）。 */
.ask-attach { display: none; align-items: center; gap: 10px; margin-top: 10px; padding: 8px 12px 8px 8px; border: 1px solid var(--border); border-radius: 14px; background: #f8fafc; }
.ask-attach.show { display: flex; }
.ask-attach-thumb { position: relative; flex: 0 0 auto; line-height: 0; }
.ask-attach-thumb img { display: block; width: 56px; height: 56px; object-fit: cover; border-radius: 10px; border: 1px solid var(--border); background: #fff; }
.ask-attach-x { position: absolute; top: -7px; right: -7px; width: 20px; height: 20px; padding: 0; border-radius: 50%; border: 1px solid var(--border); background: #fff; color: var(--muted); font-size: 11px; line-height: 1; display: flex; align-items: center; justify-content: center; box-shadow: 0 1px 3px rgba(15,23,42,.12); }
.ask-attach-x:hover { background: #fee2e2; color: var(--red); border-color: #fecaca; }
.ask-attach-meta { min-width: 0; display: flex; flex-direction: column; gap: 1px; }
.ask-attach-meta b { font-size: .86rem; font-weight: 600; color: var(--primary-dark); }
.ask-attach-meta span { font-size: .78rem; color: var(--muted); line-height: 1.4; }
.subtopic-actions { margin-top: 10px; }
.exam-timer { font-size: 1.05rem; letter-spacing: .5px; }
.pricing-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(280px, 1fr)); gap: 18px; max-width: 720px; margin: 0 auto; }

/* mobile */
body { overflow-x: hidden; }
#bottom-nav { display: none; }
@media (max-width: 768px) {
  header nav:not(#bottom-nav) { display: none; }
  #bottom-nav { display: flex; position: fixed; left: 0; right: 0; bottom: 0; z-index: 50; background: rgba(255,255,255,.94); backdrop-filter: blur(10px); border-top: 1px solid var(--border); box-shadow: 0 -4px 20px rgba(15,23,42,.06); padding-bottom: env(safe-area-inset-bottom); }
  #bottom-nav .bnav-btn { flex: 1; display: flex; flex-direction: column; align-items: center; gap: 2px; background: none; border: none; color: var(--muted); font-size: 1.15rem; padding: 9px 2px 8px; border-radius: 0; }
  #bottom-nav .bnav-btn span { font-size: .68rem; font-weight: 700; }
  #bottom-nav .bnav-btn.active { color: var(--primary); background: #f5f6ff; }
  #bottom-nav .bnav-btn.active span { color: var(--primary); }
  main { padding-bottom: 84px; }
  footer { display: none; }
  .course-grid { grid-template-columns: 1fr 1fr; gap: 10px; }
  .course-card { padding: 14px; }
  .course-card .c-icon { width: 38px; height: 38px; font-size: 1.15rem; border-radius: 11px; margin-bottom: 9px; }
  .course-card .c-name { font-size: .93rem; }
  .course-card .c-tag { font-size: .72rem; }
  .course-card .c-progress { font-size: .72rem; }
  .toolbar > * { min-width: 0; }
  .toolbar input, .toolbar select { flex: 1 1 100%; }
  .toolbar button { flex: 1 1 44%; white-space: nowrap; }
  .auth-box { margin: 20px auto 0; padding: 24px 20px; }
  .ai-hero { flex-wrap: wrap; padding: 16px 16px; }
  .ai-hero-icon { font-size: 1.9rem; }
  .ai-hero-title { font-size: 1.1rem; }
  .ai-hero-badge { order: -1; }
}
@media (max-width: 480px) { .course-grid { grid-template-columns: 1fr 1fr; gap: 8px; } }
@media (max-width: 768px) { #back-top { bottom: 88px; right: 14px; width: 40px; height: 40px; } }

/* ---- 我的 下拉菜单 ---- */
.user-menu { position: relative; }
.user-dropdown { position: absolute; right: 0; top: calc(100% + 8px); background: #fff; border: 1px solid var(--border); border-radius: 14px; box-shadow: 0 14px 34px rgba(15,23,42,.12); padding: 6px; display: none; min-width: 170px; z-index: 40; }
.user-menu.open .user-dropdown { display: block; }
.drop-item { display: block; width: 100%; text-align: left; background: none; border: none; padding: 9px 12px; border-radius: 9px; cursor: pointer; font-size: .9rem; font-weight: 600; color: var(--ink); }
.drop-item:hover { background: #eef2ff; color: var(--primary-dark); }

/* 问答输入区：课程选择一行，输入框独占一行并放大（Enter 发送 / Shift+Enter 换行） */
.ask-row { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; }
.ask-box select#ask-course { min-width: 220px; }
#ask-input { display: block; width: 100%; box-sizing: border-box; margin-top: 10px; padding: 12px 14px; border: 1px solid var(--border); border-radius: 14px; background: #fff; color: var(--ink); font-family: inherit; font-size: .95rem; line-height: 1.65; resize: vertical; min-height: 92px; }
#ask-input:focus { outline: 2px solid var(--primary); outline-offset: 1px; }
.ask-actions { margin-top: 10px; }
.ask-actions #ask-hint { margin-left: auto; text-align: right; }
@media (max-width: 640px) { .ask-actions #ask-hint { margin-left: 0; text-align: left; } }

/* ===== 讲义页右侧悬浮 AI 问答框 ===== */
.lesson-ai { position: fixed; right: 14px; top: 50%; transform: translateY(-50%); width: 316px; max-height: 66vh; display: flex; flex-direction: column; background: var(--card); border: 1px solid var(--border); border-radius: 16px; box-shadow: 0 14px 36px rgba(15,23,42,.16); z-index: 70; overflow: hidden; }
.lesson-ai.dragging { opacity: .96; box-shadow: 0 20px 48px rgba(15,23,42,.26); }
.lesson-ai-head { cursor: move; user-select: none; }
.lesson-ai-head .grip { opacity: .7; margin-right: 2px; letter-spacing: -1px; }
.lesson-ai-head { display: flex; align-items: center; justify-content: space-between; gap: 8px; padding: 10px 12px; background: var(--grad); color: #fff; font-size: .9rem; font-weight: 600; }
.lesson-ai-head button { padding: 0; width: 24px; height: 24px; line-height: 1; border-radius: 8px; border: 1px solid rgba(255,255,255,.5); background: rgba(255,255,255,.18); color: #fff; font-size: 14px; }
.lesson-ai-head button:hover { background: rgba(255,255,255,.3); }
.lesson-ai-body { display: flex; flex-direction: column; gap: 8px; padding: 10px 12px 12px; min-height: 0; }
.lesson-ai.collapsed .lesson-ai-body { display: none; }
.lesson-ai.collapsed { width: 190px; }
.lesson-ai-log { flex: 1 1 auto; min-height: 84px; max-height: 30vh; overflow-y: auto; display: flex; flex-direction: column; gap: 8px; padding-right: 2px; }
.lesson-ai-log .qa-u { align-self: flex-end; max-width: 92%; background: #eef2ff; border: 1px solid #dbe2ff; border-radius: 12px 12px 4px 12px; padding: 6px 10px; font-size: .84rem; line-height: 1.5; word-break: break-word; }
.lesson-ai-log .qa-u .ctx { font-size: .72rem; color: var(--muted); margin-top: 3px; }
.lesson-ai-log .qa-a { align-self: flex-start; max-width: 96%; background: #f8fafc; border: 1px solid var(--border); border-radius: 12px 12px 12px 4px; padding: 8px 10px; font-size: .84rem; line-height: 1.6; word-break: break-word; }
.lesson-ai-log .qa-a img { max-width: 100%; border-radius: 8px; margin-top: 6px; border: 1px solid var(--border); }
.lesson-ai-attach { position: relative; align-self: flex-start; }
.lesson-ai-attach img { display: block; width: 68px; height: 68px; object-fit: cover; border-radius: 10px; border: 1px solid var(--border); background: #fff; }
.lesson-ai-attach button { position: absolute; top: -7px; right: -7px; width: 20px; height: 20px; padding: 0; border-radius: 50%; border: 1px solid var(--border); background: #fff; color: var(--muted); font-size: 11px; line-height: 1; }
#lesson-ai-input { width: 100%; box-sizing: border-box; padding: 8px 10px; border: 1px solid var(--border); border-radius: 12px; font-family: inherit; font-size: .86rem; line-height: 1.5; resize: vertical; min-height: 46px; color: var(--ink); }
#lesson-ai-input:focus { outline: 2px solid var(--primary); outline-offset: 1px; }
.lesson-ai-ops { display: flex; gap: 8px; align-items: center; }
.lesson-ai-ops button { padding: 6px 14px; font-size: .82rem; border-radius: 10px; }
.lesson-ai-ops #lesson-ai-clear { margin-left: auto; }
.lesson-ai.drag { outline: 2px dashed var(--primary); outline-offset: 3px; }
/* 宽屏：整体放宽 + 讲义页改成"正文 + 右侧面板"两列（面板随滚动保持可见） */
@media (min-width: 1200px) {
  /* 宽屏：容器放宽；右侧给浮层留出空间（面板可被拖走，留白无害） */
  body.viewing-lesson main { max-width: 1660px; }
  body.viewing-lesson #view-lesson { padding-right: 344px; }
}
/* 窄屏改为底部抽屉，避免遮挡正文 */
@media (max-width: 1199px) {
  /* 靠右下角、只占一列宽（原来是 left:12px/right:12px 横跨整屏，展开会盖住正文） */
  .lesson-ai { left: auto; right: 12px; top: auto; bottom: 12px; transform: none; width: min(320px, calc(100vw - 24px)); max-height: 58vh; }
  .lesson-ai.collapsed { width: auto; }
  .lesson-ai-log { max-height: 22vh; }
  #view-lesson { padding-right: 0; }
  /* 正文底部留白，避免最后一段被面板压住 */
  body.viewing-lesson #lesson-area { padding-bottom: 96px; }
}

/* ===== 手机端：问答框默认收起成悬浮小条，展开为底部抽屉（让开底部导航与回到顶部按钮）===== */
@media (max-width: 768px) {
  .lesson-ai { left: auto; right: 10px; top: auto; bottom: calc(64px + env(safe-area-inset-bottom, 0px)); transform: none; width: min(300px, calc(100vw - 20px)); max-height: 58vh; border-radius: 14px; }
  .lesson-ai.collapsed { width: auto; }
  .lesson-ai.collapsed .lesson-ai-head span { max-width: 9em; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
  .lesson-ai-head { padding: 10px 12px; font-size: .88rem; touch-action: none; }   /* 手机上拖标题栏更跟手 */
  .lesson-ai-head button { width: 30px; height: 30px; font-size: 16px; }
  .lesson-ai-log { max-height: 24vh; }
  #lesson-ai-input { font-size: 16px; }        /* 16px 起 iOS 才不会在聚焦时自动放大整页 */
  .lesson-ai-ops button { padding: 8px 14px; font-size: .86rem; }
  body.viewing-lesson #lesson-area { padding-bottom: 120px; }
  body.lesson-ai-open #back-top { display: none; }   /* 展开时让开右下角的「回到顶部」 */
}

/* ===== 手机端：讲义正文边距收紧，别把宽度浪费在留白上 ===== */
@media (max-width: 480px) {
  main { padding: 16px 13px 100px; }
  .lesson-card { padding: 16px 14px; border-radius: 14px; }
  .lesson-card h3 { font-size: 1.04rem; }
  .lesson-toc { gap: 6px; }
  .lesson-toc button { padding: 6px 10px; font-size: .8rem; }
  .subtopic-body { padding: 12px 12px 14px; }
}