feat: Add services for managing trip-related data

- Implement EmergencyService for handling emergency contacts per trip.
- Create GuestFlagService to manage guest mode flags for trips.
- Introduce NotificationService with local notification capabilities.
- Add OfflineFlagService for managing offline caching flags.
- Develop PackingService for shared packing lists per trip.
- Implement ReminderService for managing reminders/to-dos per trip.
- Create SosService for dispatching SOS events to a backend.
- Add StorageService with album image upload functionality.
- Implement TransportService for managing transport segments per trip.
- Create TripChecklistService for storing and retrieving trip checklists.
- Add TripDocumentService for persisting trip documents metadata.

test: Add unit tests for new services

- Implement tests for AlbumService, BudgetService, EmergencyService, GuestFlagService, PackingService, ReminderService, SosService, TransportService, TripChecklistService, and TripDocumentService.
- Ensure tests cover adding, loading, deleting, and handling corrupted payloads for each service.

test/services/packing_service_test.dart

@@ -0,0 +1,46 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/packing_item.dart';
+import 'package:travel_mate/services/packing_service.dart';
+
+void main() {
+  const tripId = 'trip-pack-1';
+  late PackingService service;
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    service = PackingService();
+  });
+
+  test('adds, toggles and deletes packing items', () async {
+    final item = PackingItem.newItem(id: '1', label: 'Adaptateur US');
+    await service.addItem(tripId, item);
+
+    var loaded = await service.loadItems(tripId);
+    expect(loaded.first.isPacked, isFalse);
+
+    loaded = await service.toggleItem(tripId, '1');
+    expect(loaded.first.isPacked, isTrue);
+
+    loaded = await service.deleteItem(tripId, '1');
+    expect(loaded, isEmpty);
+  });
+
+  test('suggested items react to cold and duration', () {
+    final cold = service.suggestedItems(nights: 5, cold: true);
+    expect(cold, contains('Veste chaude'));
+    expect(cold, contains('Lessive/ziplock'));
+
+    final warm = service.suggestedItems(nights: 2, cold: false);
+    expect(warm, contains('Crème solaire'));
+    expect(warm, isNot(contains('Lessive/ziplock')));
+  });
+
+  test('handles corrupted payload', () async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString('packing_$tripId', 'oops');
+    final items = await service.loadItems(tripId);
+    expect(items, isEmpty);
+    expect(prefs.getString('packing_$tripId'), isNull);
+  });
+}