feat(notifications): add in-app notification center panel

- add notifications API client
- add notifications modal in chat page header
- show recent notification events with timestamps and count badge

web/src/api/notifications.ts

@@ -0,0 +1,14 @@
+import { http } from "./http";
+
+export interface NotificationItem {
+  id: number;
+  user_id: number;
+  event_type: string;
+  payload: string;
+  created_at: string;
+}
+
+export async function getNotifications(limit = 30): Promise<NotificationItem[]> {
+  const { data } = await http.get<NotificationItem[]>("/notifications", { params: { limit } });
+  return data;
+}