feat(messaging,announcements): 前端 UI 集成星标/草稿/置顶/已读回执

- 消息星标：MessageList 卡片星标按钮（乐观更新+回滚）、MessageDetail 头部星标切换
- 消息草稿：MessageCompose 自动保存（2s 防抖）+ 手动保存按钮 + 状态指示器 + 发送后清理草稿
- 公告置顶：AnnouncementCard 管理端置顶按钮、AnnouncementDetail 置顶切换、置顶 Badge
- 公告已读回执：用户端详情页自动标记已读 + 已读/未读 Badge、管理端已读人数显示
- i18n：新增 announcements.meta.readCount 翻译键

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

@@ -81,7 +81,8 @@
     "publishedAt": "Published {date}",
     "createdAt": "Created {date}",
     "updatedAt": "Updated {date}",
-    "author": "by {name}"
+    "author": "by {name}",
+    "readCount": "{count} read"
   },
   "empty": {
     "noAnnouncements": "No announcements",

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

@@ -81,7 +81,8 @@
     "publishedAt": "发布于 {date}",
     "createdAt": "创建于 {date}",
     "updatedAt": "更新于 {date}",
-    "author": "作者：{name}"
+    "author": "作者：{name}",
+    "readCount": "{count} 人已读"
   },
   "empty": {
     "noAnnouncements": "暂无公告",