feat: 重新设计公告系统为Header铃铛通知

- 新增 AnnouncementBell 组件，支持 Modal 弹窗和 Markdown 渲染
- 移除 Dashboard 横幅和独立公告页面
- 铃铛位置在 Header 文档按钮左侧，显示未读红点
- 支持点击查看详情、标记已读、全部已读等操作
- 完善国际化，移除所有硬编码中文
- 修复 AnnouncementTargetingEditor watch 循环问题

frontend/package.json

@@ -19,8 +19,10 @@
     "@vueuse/core": "^10.7.0",
     "axios": "^1.6.2",
     "chart.js": "^4.4.1",
+    "dompurify": "^3.3.1",
     "driver.js": "^1.4.0",
     "file-saver": "^2.0.5",
+    "marked": "^17.0.1",
     "pinia": "^2.1.7",
     "qrcode": "^1.5.4",
     "vue": "^3.4.0",
@@ -30,6 +32,7 @@
     "xlsx": "^0.18.5"
   },
   "devDependencies": {
+    "@types/dompurify": "^3.0.5",
     "@types/file-saver": "^2.0.7",
     "@types/mdx": "^2.0.13",
     "@types/node": "^20.10.5",