feat(client): 通知中心与未读角标(F16)

- Topbar 增加通知铃铛与未读数
- 新增 /notifications 列表页，支持筛选与标记已读
- 为 workflow/reimbursement/auth 通知提供跳转入口

client/src/router/index.tsx

@@ -7,6 +7,7 @@ import { DelegationsPage } from "../pages/DelegationsPage";
 import { ForbiddenPage } from "../pages/ForbiddenPage";
 import { InvoicePoolPage } from "../pages/InvoicePoolPage";
 import { LoginPage } from "../pages/LoginPage";
+import { NotificationsPage } from "../pages/NotificationsPage";
 import { NotFoundPage } from "../pages/NotFoundPage";
 import { ProfilePage } from "../pages/ProfilePage";
 import { ReimbursementDetailPage } from "../pages/ReimbursementDetailPage";
@@ -86,6 +87,8 @@ export function AppRouter() {
           }
         />
 
+        <Route path="notifications" element={<NotificationsPage />} />
+
         <Route
           path="audit-logs"
           element={