refactor(web): 删掉 SyncCodeEditor 的 problem prop

老实现的残留：sync.ts 拿它拼房间名 problem-${problemId}。新实现的房间以
学生为键、由服务端分配，前端不需要题号 —— 这个必填 prop 声明之后
整个组件再没用过第二次。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016DHxhKNxXfG89JnVzHbvgj

apps/web/src/oj/problem/components/ProblemEditor.vue

@@ -80,7 +80,6 @@ provide("flowchartEditorRef", flowchartEditorRef)
     <SyncCodeEditor
       v-else
       v-model:value="codeStore.code.value"
-      :problem="problem!._id"
       :language="codeStore.code.language"
       :height="editorHeight"
       @update:model-value="changeCode"

apps/web/src/shared/components/SyncCodeEditor.vue

@@ -24,7 +24,6 @@ const collabStore = useCollabStore()
 const { start, stop, getInitialExtension } = useCollabDoc()
 
 interface Props {
-  problem: string
   language?: LANGUAGE
   fontSize?: number
   height?: string