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/budget_service_test.dart

@@ -0,0 +1,51 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/budget_category.dart';
+import 'package:travel_mate/services/budget_service.dart';
+
+void main() {
+  const tripId = 'trip-budget-1';
+  late BudgetService service;
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    service = BudgetService();
+  });
+
+  test('adds and deletes budget envelopes', () async {
+    final cat = BudgetCategory.newCategory(
+      id: 'food',
+      name: 'Food',
+      planned: 300,
+      currency: 'EUR',
+    );
+    await service.addBudget(tripId, cat);
+
+    var loaded = await service.loadBudgets(tripId);
+    expect(loaded.single.name, 'Food');
+
+    loaded = await service.deleteBudget(tripId, 'food');
+    expect(loaded, isEmpty);
+  });
+
+  test('updates spent amount', () async {
+    final cat = BudgetCategory.newCategory(
+      id: 'transport',
+      name: 'Transport',
+      planned: 200,
+      currency: 'USD',
+    );
+    await service.addBudget(tripId, cat);
+
+    final updated = await service.updateSpent(tripId, 'transport', 55.5);
+    expect(updated.first.spent, 55.5);
+  });
+
+  test('corrupted payload cleared', () async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString('budget_$tripId', 'oops');
+    final loaded = await service.loadBudgets(tripId);
+    expect(loaded, isEmpty);
+    expect(prefs.getString('budget_$tripId'), isNull);
+  });
+}