feat(web): 协作编辑走 collab 通道

Yjs sync/awareness 协议直接跑在 /ws/collab 上，服务端哑转发。
内容源是学生：学生端先把编辑器内容写进 ytext 再挂 yCollab，
教师端 seedContent 恒为 null —— 这条根治了老实现里谁的代码活下来看运气的问题。

订正 brief 里的一处疏漏：SyncCodeEditor.vue 挂在每个用户的题目页上，教师也不例外
（ProblemEditor.vue 只按语言是不是 Flowchart 分支，不看角色）。教师接单同样会让
collabStore.room 非空，若不按角色收窄，教师自己停在某道题页面上接单时，这个组件
会把教师自己的编辑器内容当成种子插入文档，还会跟 CollabModal 抢
setBinaryHandler 这个单例槽位。改为 `room && !collabStore.isTeacher` 才起协作，
教师端的协作只归 CollabModal 管。

另外两处修正：
- editorView 用 shallowRef 而非 ref —— CodeMirror 的 EditorView 是带 getter 的类
  实例，ref() 的深度 UnwrapRef 会把它拆成丢了原型方法的假类型，vue-tsc 报错。
- Header.vue 挂 CollabModal 放进根 n-flex 内部而不是同级 —— 组件一旦变成多根
  fragment，default.vue 里 `<Header class="header" />` 的 class 就没有单一根节点
  可以落地，header 行会丢掉居中样式（实测触发了 Vue 的
  Extraneous non-props attributes 警告）。n-modal 默认 teleport 到 body，塞在
  这里不影响其实际渲染位置。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1d8B3f4SXJwDvUY625eQd

bun.lock

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