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/shared/i18n/messages/zh-CN/announcements.json

@@ -0,0 +1,90 @@
+{
+  "title": {
+    "list": "公告",
+    "detail": "公告详情",
+    "adminList": "公告管理",
+    "edit": "编辑公告",
+    "create": "新建公告"
+  },
+  "description": {
+    "list": "随时了解学校最新公告动态。",
+    "adminList": "创建并管理全校公告。",
+    "edit": "更新公告详情。"
+  },
+  "filter": {
+    "all": "全部",
+    "published": "已发布",
+    "draft": "草稿",
+    "archived": "已归档",
+    "placeholder": "按状态筛选"
+  },
+  "status": {
+    "draft": "草稿",
+    "published": "已发布",
+    "archived": "已归档"
+  },
+  "type": {
+    "school": "全校",
+    "grade": "年级",
+    "class": "班级"
+  },
+  "form": {
+    "title": "标题",
+    "titlePlaceholder": "公告标题",
+    "content": "内容",
+    "contentPlaceholder": "请输入公告内容...",
+    "type": "类型",
+    "typePlaceholder": "选择类型",
+    "status": "状态",
+    "statusPlaceholder": "选择状态",
+    "targetGrade": "目标年级",
+    "targetGradePlaceholder": "选择年级（可选）",
+    "targetClass": "目标班级",
+    "targetClassPlaceholder": "选择班级（可选）",
+    "submit": "保存",
+    "create": "创建",
+    "cancel": "取消",
+    "saving": "保存中..."
+  },
+  "actions": {
+    "new": "新建公告",
+    "publish": "发布",
+    "archive": "归档",
+    "edit": "编辑",
+    "delete": "删除",
+    "back": "返回"
+  },
+  "messages": {
+    "created": "公告已创建",
+    "updated": "公告已更新",
+    "deleted": "公告已删除",
+    "published": "公告已发布",
+    "archived": "公告已归档",
+    "notFound": "公告不存在",
+    "createFailed": "创建公告失败",
+    "updateFailed": "保存公告失败",
+    "publishFailed": "发布失败",
+    "archiveFailed": "归档失败",
+    "deleteFailed": "删除失败",
+    "invalidForm": "表单数据无效",
+    "invalidState": "表单状态无效"
+  },
+  "meta": {
+    "publishedAt": "发布于 {date}",
+    "createdAt": "创建于 {date}",
+    "updatedAt": "更新于 {date}",
+    "author": "作者：{name}"
+  },
+  "empty": {
+    "noAnnouncements": "暂无公告",
+    "noAnnouncementsDesc": "目前还没有任何公告。",
+    "noMatch": "当前筛选条件下暂无公告。",
+    "deleteTitle": "删除公告",
+    "deleteDesc": "此操作将永久删除"{title}"。"
+  },
+  "error": {
+    "loadFailed": "公告加载失败",
+    "loadFailedDesc": "抱歉，加载公告时发生了意外错误。请稍后重试。",
+    "retry": "重试"
+  }
+}