feat(announcements,messaging): 公告与消息模块审计重构 — i18n + Error Boundary + a11y

- 新增审计报告 docs/architecture/audit/announcements-messages-audit-report.md
- 新增中英双语 i18n 字典 announcements.json / messages.json（11/13 个命名空间）
- 重构所有 announcements 和 messaging 组件接入 next-intl（useTranslations）
- 所有页面 page.tsx 使用 generateMetadata + getTranslations 替代硬编码 metadata
- 新增 7 个 error.tsx 错误边界（4 公告 + 3 消息），统一 EmptyState + i18n + 重试
- a11y 改进：announcement-card / message-list / notification-dropdown 添加 aria-label
- 同步架构图 004 和 005：i18n.messages 清单 + 已知问题修复记录

src/app/(dashboard)/admin/announcements/page.tsx

@@ -1,5 +1,6 @@
-import type { Metadata } from "next"
+import type { Metadata } from "next"
 import type { JSX } from "react"
+import { getTranslations } from "next-intl/server"
 
 import { requirePermission } from "@/shared/lib/auth-guard"
 import { Permissions } from "@/shared/types/permissions"
@@ -10,13 +11,16 @@ import { AdminAnnouncementsView } from "@/modules/announcements/components/admin
 import { getSearchParam, type SearchParams } from "@/shared/lib/utils"
 import type { AnnouncementStatus } from "@/modules/announcements/types"
 
-export const metadata: Metadata = {
-  title: "公告管理 - Next_Edu",
-  description: "管理系统公告，支持草稿、发布与归档",
-}
-
 export const dynamic = "force-dynamic"
 
+export async function generateMetadata(): Promise<Metadata> {
+  const t = await getTranslations("announcements")
+  return {
+    title: t("title.adminList"),
+    description: t("description.adminList"),
+  }
+}
+
 const isValidStatus = (v?: string): v is AnnouncementStatus =>
   v === "draft" || v === "published" || v === "archived"