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/guest_flag_service_test.dart

@@ -0,0 +1,19 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/services/guest_flag_service.dart';
+
+void main() {
+  const tripId = 'trip-guest-1';
+  late GuestFlagService service;
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    service = GuestFlagService();
+  });
+
+  test('sets and reads guest mode flag', () async {
+    expect(await service.isGuestEnabled(tripId), isFalse);
+    await service.setGuestEnabled(tripId, true);
+    expect(await service.isGuestEnabled(tripId), isTrue);
+  });
+}