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/trip_document_service_test.dart

@@ -0,0 +1,60 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/trip_document.dart';
+import 'package:travel_mate/services/trip_document_service.dart';
+
+void main() {
+  const tripId = 'trip-docs-1';
+  late TripDocumentService service;
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    service = TripDocumentService();
+  });
+
+  test('adds and loads documents', () async {
+    final doc = TripDocument.newEntry(
+      id: 'doc1',
+      title: 'Billet Aller',
+      category: 'billet',
+      downloadUrl: 'https://example.com/billet.pdf',
+    );
+
+    await service.addDocument(tripId, doc);
+    final loaded = await service.loadDocuments(tripId);
+
+    expect(loaded, hasLength(1));
+    expect(loaded.first.title, 'Billet Aller');
+    expect(loaded.first.category, 'billet');
+  });
+
+  test('deletes a document', () async {
+    final a = TripDocument.newEntry(
+      id: 'a',
+      title: 'Passeport',
+      category: 'passeport',
+    );
+    final b = TripDocument.newEntry(
+      id: 'b',
+      title: 'Assurance',
+      category: 'assurance',
+    );
+    await service.addDocument(tripId, a);
+    await service.addDocument(tripId, b);
+
+    final afterDelete = await service.deleteDocument(tripId, 'a');
+
+    expect(afterDelete, hasLength(1));
+    expect(afterDelete.first.id, 'b');
+  });
+
+  test('handles corrupted payload gracefully', () async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString('trip_docs_$tripId', 'oops');
+
+    final docs = await service.loadDocuments(tripId);
+
+    expect(docs, isEmpty);
+    expect(prefs.getString('trip_docs_$tripId'), isNull);
+  });
+}