feat: expose user activity timestamps in admin list

frontend/src/types/index.ts

@@ -47,6 +47,8 @@ export interface User {
   balance_notify_threshold: number | null
   balance_notify_extra_emails: NotifyEmailEntry[]
   subscriptions?: UserSubscription[] // User's active subscriptions
+  last_login_at?: string | null
+  last_active_at?: string | null
   created_at: string
   updated_at: string
 }