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/components/layout/AppHeader.vue

@@ -21,8 +21,11 @@
         </div>
       </div>
 
-      <!-- Right: Docs + Language + Subscriptions + Balance + User Dropdown -->
+      <!-- Right: Announcements + Docs + Language + Subscriptions + Balance + User Dropdown -->
       <div class="flex items-center gap-3">
+        <!-- Announcement Bell -->
+        <AnnouncementBell v-if="user" />
+
         <!-- Docs Link -->
         <a
           v-if="docUrl"
@@ -210,6 +213,7 @@ import { useI18n } from 'vue-i18n'
 import { useAppStore, useAuthStore, useOnboardingStore } from '@/stores'
 import LocaleSwitcher from '@/components/common/LocaleSwitcher.vue'
 import SubscriptionProgressMini from '@/components/common/SubscriptionProgressMini.vue'
+import AnnouncementBell from '@/components/common/AnnouncementBell.vue'
 import Icon from '@/components/icons/Icon.vue'
 
 const router = useRouter()