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/AppSidebar.vue

@@ -433,7 +433,6 @@ const ChevronDoubleRightIcon = {
 const userNavItems = computed(() => {
   const items = [
     { path: '/dashboard', label: t('nav.dashboard'), icon: DashboardIcon },
-    { path: '/announcements', label: t('nav.announcements'), icon: BellIcon },
     { path: '/keys', label: t('nav.apiKeys'), icon: KeyIcon },
     { path: '/usage', label: t('nav.usage'), icon: ChartIcon, hideInSimpleMode: true },
     { path: '/subscriptions', label: t('nav.mySubscriptions'), icon: CreditCardIcon, hideInSimpleMode: true },
@@ -456,7 +455,6 @@ const userNavItems = computed(() => {
 // Personal navigation items (for admin's "My Account" section, without Dashboard)
 const personalNavItems = computed(() => {
   const items = [
-    { path: '/announcements', label: t('nav.announcements'), icon: BellIcon },
     { path: '/keys', label: t('nav.apiKeys'), icon: KeyIcon },
     { path: '/usage', label: t('nav.usage'), icon: ChartIcon, hideInSimpleMode: true },
     { path: '/subscriptions', label: t('nav.mySubscriptions'), icon: CreditCardIcon, hideInSimpleMode: true },