feat: implement offline synchronization with mutation queue and background sync

- Created useOfflineSync hook for connectivity detection and sync management
- Implemented localStorage-based mutation queue for offline operations
- Added background sync support in service worker
- Created OfflineIndicator component to show connection status
- Added offline-aware action wrappers for seamless offline/online transitions
- Implemented conflict handling with retry logic (up to 3 retries)
- Auto-sync when connection is restored
- Validates Requirements 4.5

hooks/README.md

@@ -188,10 +188,73 @@ function MyComponent({ listId }) {
 }
 ```
 
+### `useOfflineSync`
+
+Manages offline synchronization with automatic mutation queuing and background sync.
+
+**Features:**
+- Automatic online/offline detection
+- Mutation queue for offline operations
+- Background sync when connection restored
+- Conflict handling
+- Retry logic with exponential backoff
+
+**Usage:**
+```typescript
+import { useOfflineSync } from '@/hooks'
+
+function MyComponent() {
+  const { 
+    isOnline, 
+    isSyncing, 
+    syncError, 
+    queueLength, 
+    queueMutation, 
+    triggerSync,
+    clearQueue 
+  } = useOfflineSync()
+
+  // Queue a mutation when offline
+  const handleCreateItem = async () => {
+    if (!isOnline) {
+      queueMutation({
+        type: 'create',
+        entity: 'item',
+        data: { listId: '123', title: 'New Item' }
+      })
+    } else {
+      await createItem('123', 'New Item')
+    }
+  }
+
+  return (
+    <div>
+      {!isOnline && <div>Offline - changes will sync when online</div>}
+      {isSyncing && <div>Syncing {queueLength} changes...</div>}
+      {syncError && <div>Sync error: {syncError.message}</div>}
+    </div>
+  )
+}
+```
+
+**Offline-Aware Actions:**
+
+For convenience, use the offline-aware action wrappers from `lib/offline-actions.ts`:
+
+```typescript
+import { createItemOffline, updateItemOffline, deleteItemOffline } from '@/lib/offline-actions'
+
+// These automatically queue mutations when offline
+await createItemOffline(listId, title, description, metadata)
+await updateItemOffline(itemId, updates)
+await deleteItemOffline(itemId)
+```
+
 ## Requirements Validation
 
 These hooks implement the following requirements from the specification:
 
+- **Requirement 4.5**: Offline synchronization when connectivity restored
 - **Requirement 6.4**: Filter state management and clearing
 - **Requirement 7.3**: Visual feedback for user actions (optimistic updates)
 - **Requirement 7.4**: Scroll position preservation