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/src/i18n/locales/en.ts

@@ -3139,13 +3139,22 @@ export default {
     description: 'View system announcements',
     unreadOnly: 'Show unread only',
     markRead: 'Mark as read',
+    markAllRead: 'Mark all as read',
+    viewAll: 'View all announcements',
+    markedAsRead: 'Marked as read',
+    allMarkedAsRead: 'All announcements marked as read',
+    newCount: '{count} new announcement | {count} new announcements',
     readAt: 'Read at',
     read: 'Read',
     unread: 'Unread',
     startsAt: 'Starts at',
     endsAt: 'Ends at',
     empty: 'No announcements',
-    emptyUnread: 'No unread announcements'
+    emptyUnread: 'No unread announcements',
+    total: 'announcements',
+    emptyDescription: 'There are no system announcements at this time',
+    readStatus: 'You have read this announcement',
+    markReadHint: 'Click "Mark as read" to mark this announcement'
   },
 
   // User Subscriptions Page