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/album_service_test.dart

@@ -0,0 +1,39 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/album_photo.dart';
+import 'package:travel_mate/services/album_service.dart';
+
+void main() {
+  const tripId = 'trip-album-1';
+  late AlbumService service;
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    service = AlbumService();
+  });
+
+  test('adds and loads photos', () async {
+    final photo = AlbumPhoto.newPhoto(
+      id: 'p1',
+      url: 'https://example.com/img.jpg',
+      caption: 'Coucher de soleil',
+      uploadedBy: 'Alice',
+    );
+    await service.addPhoto(tripId, photo);
+    final loaded = await service.loadPhotos(tripId);
+    expect(loaded.single.url, contains('example.com'));
+  });
+
+  test('deletes photo and clears corrupted payload', () async {
+    final p = AlbumPhoto.newPhoto(id: 'p1', url: 'u', caption: null);
+    await service.addPhoto(tripId, p);
+    var updated = await service.deletePhoto(tripId, 'p1');
+    expect(updated, isEmpty);
+
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString('album_$tripId', 'oops');
+    updated = await service.loadPhotos(tripId);
+    expect(updated, isEmpty);
+    expect(prefs.getString('album_$tripId'), isNull);
+  });
+}