feat: Introduce offline synchronization for time registrations via a new sync service.

lib/main.dart

@@ -4,6 +4,8 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:hive_flutter/hive_flutter.dart';
 import 'screens/auth/login_screen.dart';
 import 'screens/home/home_screen.dart';
+import 'services/sync_service.dart';
+import 'providers/time_provider.dart';
 import 'providers/auth_provider.dart';
 
 void main() async {
@@ -15,9 +17,16 @@ void main() async {
   // Initialiser Hive for offline støtte
   await Hive.initFlutter();
   
+  // Initialiser SyncService
+  final syncService = SyncService();
+  await syncService.init();
+  
   runApp(
-    const ProviderScope(
-      child: TimeRegApp(),
+    ProviderScope(
+      overrides: [
+        syncServiceProvider.overrideWithValue(syncService),
+      ],
+      child: const TimeRegApp(),
     ),
   );
 }