refactor(web): 删掉 y-webrtc 协同编辑的旧实现

sync.ts / syncStatus.ts、y-webrtc 依赖、PUBLIC_SIGNALING_URL 全部移除，
外部信令服务器 signaling.xuyue.cc 不再被使用。
顺带修掉 CLAUDE.md 里「协作在流程图编辑器」这句一直是错的描述。

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

apps/web/src/oj/composables/syncStatus.ts

@@ -1,73 +0,0 @@
-import { ref, provide, inject } from "vue"
-
-/**
- * 同步状态管理 composable
- * 使用 provide/inject 模式在组件树中共享状态
- */
-
-export interface SyncStatusState {
-  hadConnection: boolean
-  otherUser?: { name: string; isSuperAdmin: boolean }
-  lastLeftUser?: { name: string; isSuperAdmin: boolean } // 保存离开之人的信息
-}
-
-// 提供/注入的 key
-export const SYNC_STATUS_KEY = Symbol("syncStatus")
-
-/**
- * 创建同步状态实例
- * 每次调用创建新的状态实例
- */
-export function createSyncStatus() {
-  const otherUser = ref<{ name: string; isSuperAdmin: boolean }>()
-  const hadConnection = ref(false)
-  const lastLeftUser = ref<{ name: string; isSuperAdmin: boolean }>()
-
-  const setOtherUser = (user?: { name: string; isSuperAdmin: boolean }) => {
-    // 如果之前有其他用户，现在没有了，说明用户离开了
-    if (otherUser.value && !user) {
-      lastLeftUser.value = otherUser.value
-    }
-    otherUser.value = user
-    if (user) {
-      hadConnection.value = true
-    }
-  }
-
-  const reset = () => {
-    otherUser.value = undefined
-    hadConnection.value = false
-    lastLeftUser.value = undefined
-  }
-
-  return {
-    otherUser,
-    hadConnection,
-    lastLeftUser,
-    setOtherUser,
-    reset,
-  }
-}
-
-/**
- * 提供同步状态到子组件
- * 在父组件中调用
- */
-export function provideSyncStatus() {
-  const syncStatus = createSyncStatus()
-  provide(SYNC_STATUS_KEY, syncStatus)
-  return syncStatus
-}
-
-/**
- * 注入同步状态
- * 在子组件中调用，获取父组件提供的状态
- */
-export function injectSyncStatus() {
-  const syncStatus =
-    inject<ReturnType<typeof createSyncStatus>>(SYNC_STATUS_KEY)
-  if (!syncStatus) {
-    throw new Error("syncStatus must be provided by a parent component")
-  }
-  return syncStatus
-}

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

@@ -5,7 +5,6 @@ import { useProblemStore } from "oj/store/problem"
 import { SOURCES } from "utils/constants"
 import CodeEditor from "shared/components/CodeEditor.vue"
 import { useBreakpoints } from "shared/composables/breakpoints"
-import { provideSyncStatus } from "oj/composables/syncStatus"
 import storage from "utils/storage"
 import type { LANGUAGE } from "utils/types"
 import Form from "./Form.vue"
@@ -18,10 +17,6 @@ const { problem } = storeToRefs(problemStore)
 
 const { isDesktop } = useBreakpoints()
 
-// 提供空的同步状态，避免 Form 组件注入错误
-// 在竞赛模式下，同步功能会被 showSyncFeature 自动禁用
-provideSyncStatus()
-
 const contestID = route.params.contestID || null
 const storageKey = computed(
   () =>