feat(admin): A6 通知页前端 — 统一通知/事件中心(管理面板#16)

- Notifications.tsx(254行): 类型筛选(全部/system/channel/backthink/audit,服务端重拉) + before游标加载更多(id去重/到底判定) + 空态/错误(后端文案) + 30s静默刷新; 卡片行(非AdminTable,避其rowKey升序与倒序feed冲突); Miuix
- api.ts adminListNotifications + types.ts AdminNotification(Query) + sections.tsx 侧栏项{id:notifications,group:运行时,icon:light} + AdminApp renderSection + router.ts VALID_SECTIONS
- 契约: GET /admin/notifications?type=&before=&limit= → {data:[{id,type,title,body,source,createdAt}]}; 未读/清空需后端read端点(诚实Notice,不做前端假实现)
- 前端 build 绿(157 modules); 后端端点由并发代理[194]落地
- icon 用现有 light(Icon.tsx 无 bell; 如需铃铛另加 path)

frontend/web/src/admin/router.ts

@@ -20,6 +20,7 @@ export type AdminSectionId =
   | "agents"
   | "flow"
   | "backthink"
+  | "notifications"
   | "observability"
   | "system"
   | "users";
@@ -43,6 +44,7 @@ const VALID_SECTIONS: ReadonlySet<string> = new Set([
   "agents",
   "flow",
   "backthink",
+  "notifications",
   "observability",
   "system",
   "users",