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/router/index.ts

@@ -187,18 +187,6 @@ const routes: RouteRecordRaw[] = [
       descriptionKey: 'purchase.description'
     }
   },
-  {
-    path: '/announcements',
-    name: 'Announcements',
-    component: () => import('@/views/user/AnnouncementsView.vue'),
-    meta: {
-      requiresAuth: true,
-      requiresAdmin: false,
-      title: 'Announcements',
-      titleKey: 'announcements.title',
-      descriptionKey: 'announcements.description'
-    }
-  },
 
   // ==================== Admin Routes ====================
   {