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

components/ListDetailView.tsx

@@ -11,6 +11,7 @@ import ShareListModal from './ShareListModal'
 import PermissionBadge from './PermissionBadge'
 import { searchItems, getItemsByFilter } from '@/lib/actions'
 import { useSearch, useFilters, useScrollPosition } from '@/hooks'
+import { useRealtimeSync } from '@/hooks/useRealtimeSync'
 
 interface Item {
   id: string
@@ -73,6 +74,13 @@ export default function ListDetailView({
   // Use scroll position preservation hook
   useScrollPosition({ key: `list-${listId}` })
 
+  // Use real-time sync hook to poll for updates
+  useRealtimeSync({
+    listId,
+    enabled: true,
+    pollInterval: 3000, // Poll every 3 seconds
+  })
+
   // Extract all unique tags from items
   const availableTags = useMemo(() => {
     const tagSet = new Set<string>()