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

app/layout.tsx

@@ -1,6 +1,7 @@
 import type { Metadata } from 'next'
 import './globals.css'
 import ServiceWorkerRegistration from '@/components/ServiceWorkerRegistration'
+import OfflineIndicator from '@/components/OfflineIndicator'
 
 export const metadata: Metadata = {
   title: 'Ultimate Lists',
@@ -36,6 +37,7 @@ export default function RootLayout({
       </head>
       <body className="bg-gray-50 min-h-screen">
         <ServiceWorkerRegistration />
+        <OfflineIndicator />
         {children}
       </body>
     </html>