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>

bun.lock

@@ -12,6 +12,8 @@
         "@codemirror/lang-css": "^6.3.1",
         "@codemirror/lang-html": "^6.4.12",
         "@codemirror/lang-javascript": "^6.2.5",
+        "@codemirror/language": "6.12.3",
+        "@codemirror/theme-one-dark": "^6.1.3",
         "@codemirror/view": "^6.41.1",
         "@fontsource/jetbrains-mono": "^5.3.0",
         "@vueuse/core": "^15.0.0",
@@ -295,6 +297,8 @@
 
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+    "@codemirror/theme-one-dark": ["@codemirror/theme-one-dark@6.1.3", "", { "dependencies": { "@codemirror/language": "^6.0.0", "@codemirror/state": "^6.0.0", "@codemirror/view": "^6.0.0", "@lezer/highlight": "^1.0.0" } }, "sha512-NzBdIvEJmx6fjeremiGp3t/okrLPYT0d9orIc7AFun8oZcRk58aejkqhv6spnz4MLAevrKNPMQYXEWMg4s+sKA=="],
+
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