refactor(announcements,messaging,notifications): V1+V2 审计重构 — i18n 命名空间独立 + 通知标题 i18n 化 + 服务端过滤 + 编排下沉 + 表单错误展示 + 架构图同步

V1 改进（已完成）:
- P0-4/P1-4/P1-5: 通知组件和 CRUD Action 从 messaging 迁移至 notifications 模块
- P1-5: 新增 getMessagesPageData / getAdminAnnouncementsPageData 编排函数
- P1-6: announcements schema 添加 superRefine 条件校验
- P1-7: 新增 useMessageSearch hook（防抖 + 请求竞态取消）+ 客户端分页 UI
- P1-9: deleteMessage 事务化
- P2-11: 全模块 trackEvent 埋点
- 全模块 i18n 接入 + Error Boundary + a11y 改进

V2 改进（本次完成）:
- V2-P0-1: 通知 i18n 命名空间独立（notifications.json），useTranslations 从 "messages" 切换到 "notifications"
- V2-P0-2: 公告/消息通知标题 i18n 化，Server Action 中使用 getTranslations 生成通知标题
- V2-P1-1: AnnouncementList 纯服务端过滤，移除客户端 useState/useMemo
- V2-P1-2: MessageList 客户端过滤仅在初始数据时执行，搜索结果由服务端按 tab 过滤
- V2-P1-3: 消息详情页编排下沉，新增 getMessageDetailPageData 编排函数
- V2-P1-4: 表单服务端校验错误展示（fieldErrors + aria-invalid）
- V2-P2-1: 轮询间隔常量化（POLL_INTERVAL_MS）
- V2-P2-2: 架构图同步（004 + 005）

src/shared/i18n/messages/en/notifications.json

@@ -0,0 +1,39 @@
+{
+  "title": {
+    "list": "Notifications",
+    "dropdown": "Notifications"
+  },
+  "description": {
+    "list": "Stay updated on your latest activities."
+  },
+  "actions": {
+    "markRead": "Mark as read",
+    "markAllRead": "Mark all as read",
+    "viewAll": "View all notifications",
+    "view": "View"
+  },
+  "type": {
+    "message": "Message",
+    "announcement": "Announcement",
+    "homework": "Homework",
+    "grade": "Grade"
+  },
+  "status": {
+    "new": "New"
+  },
+  "empty": {
+    "noNotifications": "No notifications",
+    "noNotificationsDesc": "You have no notifications yet.",
+    "noNotificationsDropdown": "No notifications"
+  },
+  "messages": {
+    "allMarkedRead": "All notifications marked as read",
+    "notificationMarkedRead": "Notification marked as read",
+    "markReadFailed": "Failed to mark as read"
+  },
+  "error": {
+    "loadFailed": "Failed to load notifications",
+    "loadFailedDesc": "Sorry, an unexpected error occurred while loading notifications. Please try again later.",
+    "retry": "Retry"
+  }
+}