feat: implement real-time list synchronization

- Add useRealtimeSync hook for polling server updates every 3 seconds
- Create API endpoint to check last updated timestamp for lists
- Integrate real-time sync into ListDetailView component
- Automatically refresh when other users make changes to shared lists
- Poll on interval and when page becomes visible again

hooks/index.ts

@@ -10,3 +10,4 @@ export { useOptimisticUpdate, useOptimisticList } from './useOptimisticUpdate'
 export { useScrollPosition, useContainerScrollPosition } from './useScrollPosition'
 export { useOfflineSync } from './useOfflineSync'
 export type { QueuedMutation } from './useOfflineSync'
+export { useRealtimeSync } from './useRealtimeSync'