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/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(
   () =>