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/[id]/page.tsx

@@ -1,6 +1,7 @@
 import { notFound } from "next/navigation"
 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"
@@ -8,13 +9,16 @@ import { getAnnouncementById } from "@/modules/announcements/data-access"
 import { getGrades } from "@/modules/school/data-access"
 import { AnnouncementForm } from "@/modules/announcements/components/announcement-form"
 
-export const metadata: Metadata = {
-  title: "编辑公告 - Next_Edu",
-  description: "更新公告详情",
-}
-
 export const dynamic = "force-dynamic"
 
+export async function generateMetadata(): Promise<Metadata> {
+  const t = await getTranslations("announcements")
+  return {
+    title: t("title.edit"),
+    description: t("description.edit"),
+  }
+}
+
 export default async function EditAnnouncementPage({
   params,
 }: {
@@ -22,6 +26,7 @@ export default async function EditAnnouncementPage({
 }): Promise<JSX.Element> {
   await requirePermission(Permissions.ANNOUNCEMENT_MANAGE)
   const { id } = await params
+  const t = await getTranslations("announcements")
 
   const [announcement, grades] = await Promise.all([
     getAnnouncementById(id),
@@ -33,8 +38,8 @@ export default async function EditAnnouncementPage({
   return (
     <div className="flex h-full flex-col space-y-8 p-8">
       <div>
-        <h2 className="text-2xl font-bold tracking-tight">编辑公告</h2>
-        <p className="text-muted-foreground">更新公告详情。</p>
+        <h2 className="text-2xl font-bold tracking-tight">{t("title.edit")}</h2>
+        <p className="text-muted-foreground">{t("description.edit")}</p>
       </div>
       <AnnouncementForm
         mode="edit"