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)/announcements/page.tsx

@@ -1,4 +1,5 @@
 import type { Metadata } from "next"
+import { getTranslations } from "next-intl/server"
 
 import { requirePermission } from "@/shared/lib/auth-guard"
 import { Permissions } from "@/shared/types/permissions"
@@ -12,8 +13,9 @@ import {
 
 export const dynamic = "force-dynamic"
 
-export const metadata: Metadata = {
-  title: "Announcements",
+export async function generateMetadata(): Promise<Metadata> {
+  const t = await getTranslations("announcements")
+  return { title: t("title.list") }
 }
 
 /**
@@ -76,6 +78,7 @@ async function resolveAudience(ctx: {
 }
 
 export default async function AnnouncementsPage() {
+  const t = await getTranslations("announcements")
   const ctx = await requirePermission(Permissions.ANNOUNCEMENT_READ)
   const audience = await resolveAudience(ctx)
 
@@ -87,9 +90,9 @@ export default async function AnnouncementsPage() {
   return (
     <div className="flex h-full flex-col space-y-8 p-8">
       <div>
-        <h2 className="text-2xl font-bold tracking-tight">Announcements</h2>
+        <h2 className="text-2xl font-bold tracking-tight">{t("title.list")}</h2>
         <p className="text-muted-foreground">
-          Stay up to date with the latest school announcements.
+          {t("description.list")}
         </p>
       </div>
       <AnnouncementList