feat: 改版阶段 4：挑战列表页、教程按小节存代码、出题页合并、用户管理、整站暗色

- 挑战列表改成独立页面（ChallengeHome）：每题一行，完成状态、分值、出题人，可按未做 / 已交筛选；
  教程面板只放教程
- 教程每一节的代码各存一份（和挑战同一个按任务的 key），切回来时还在，第一次才放示例代码；
  提交列表“复制到编辑器”对教程也写入该任务的代码
- 教程 / 挑战出题页合并为 TaskEditor：“提交”改为“保存”，保存后停在当前项，删除前说明会连带删除学生提交；
  admin 也能在列表里切换挑战是否公开（保存时本来就能设）
- 用户管理：初始密码默认隐藏，封号前确认，批量新建说明用户名规则
- 整站暗色：顶栏切换跟随系统 / 亮色 / 暗色；Naive 暗色主题、markdown 和代码高亮的暗色覆盖
  （styles/dark-content.css 由 GitHub 暗色样式生成）、CodeMirror One Dark 配色、md-editor 暗色；
  组件里剩下的硬编码颜色全部换成配色变量

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -28,9 +28,10 @@ The backend was rewritten from Django in 2026-10 to cut memory use (~25 MB vs. 1
   - `src/token.ts` — tokens in localStorage; refreshes the access token shortly before expiry (one request at a time) and builds WebSocket URLs
   - `src/api.ts` — Axios-based API client; adds the Bearer header, on 401 refreshes once and retries, otherwise clears login and goes home; 403 alerts and goes home
   - `src/router.ts` — Vue Router; `meta.auth` needs login, `meta.role` (`staff` = admin+super, `super`) is checked in the guard after `store/session.loadUser()`, `meta.bare` pages render without the top bar
-  - `src/theme.ts` — the only place colors are defined: one palette (light + dark values) becomes both `--wp-*` CSS variables and Naive UI `themeOverrides`; use `var(--wp-...)` in styles, never hard-coded colors. No `color-mix()` (Chrome 105)
+  - `src/theme.ts` — the only place colors are defined: one palette (light + dark values) becomes both `--wp-*` CSS variables and Naive UI `themeOverrides`; use `var(--wp-...)` in styles, never hard-coded colors (iframes stay white: they show students' pages). No `color-mix()` (Chrome 105). `store/theme.ts` holds the mode (auto / light / dark, toggled in the top bar) and sets `data-theme` on `<html>`; `styles/dark-content.css` is generated from the GitHub dark markdown and highlight.js styles (selectors prefixed with `:root[data-theme="dark"]`); CodeMirror uses One Dark highlighting in dark mode
   - `src/icons.ts` — `<Icon icon="lucide:name" />` renders bundled `lucide-vue-next` icons (works offline); register new names in its map. Fonts are bundled too (`@fontsource/jetbrains-mono`)
-  - `components/workbench/Workbench.vue` — shared challenge / creation page: left = brief + AI chat (`PromptPanel`), right = preview / code (`CodeTabs`) + `VersionBar`. Every assistant message with code is a version; hand edits and pasted external-AI code are saved as versions via `POST /prompt/versions` (`source` `edit` / `manual`, stored as a user+assistant pair). Challenges are submitted only when the student clicks “交这一版” (`POST /submission/` with `message_id`), never automatically. Editor code is per task (`composables/useTaskCode.ts`, localStorage `web-code:<user>:<task>`); the tutorial page still uses the shared `store/editors.ts`
+  - `components/workbench/Workbench.vue` — shared challenge / creation page: left = brief + AI chat (`PromptPanel`), right = preview / code (`CodeTabs`) + `VersionBar`. Every assistant message with code is a version; hand edits and pasted external-AI code are saved as versions via `POST /prompt/versions` (`source` `edit` / `manual`, stored as a user+assistant pair). Challenges are submitted only when the student clicks “交这一版” (`POST /submission/` with `message_id`), never automatically. Editor code is per task (`composables/useTaskCode.ts`, localStorage `web-code:<user>:<task>`); the tutorial page edits the shared `store/editors.ts` refs but `Workspace.vue` saves them under the same per-task key and `TutorialContent` restores them (example code only the first time)
+  - `pages/ChallengeHome.vue` — challenge list page; `pages/TaskEditor.vue` (prop `kind`) is the single tutorial / challenge authoring page
   - `pages/Review.vue` (“批改”, default teaching page) + `components/review/ReviewPanel.vue` (also opened from the gradebook) — keys: J/K next/prev student, 1–5 rate, P present; `components/review/PeersDrawer.vue` is the students' “同学作品”
   - `components/layout/AppShell.vue` — top bar for every page; `pages/Dashboard.vue` is the teaching sidebar (“教学”) under `/dashboard`
   - `src/store/` — Reactive state (plain Vue refs, not Vuex/Pinia); `prompt.ts` / `guidance.ts` own the WebSockets