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.

lib/services/guest_flag_service.dart

@@ -0,0 +1,18 @@
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Stores a simple read-only guest mode flag per trip.
+class GuestFlagService {
+  /// Returns whether guest mode is enabled for [tripId].
+  Future<bool> isGuestEnabled(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    return prefs.getBool(_key(tripId)) ?? false;
+  }
+
+  /// Sets guest mode flag for [tripId].
+  Future<void> setGuestEnabled(String tripId, bool enabled) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setBool(_key(tripId), enabled);
+  }
+
+  String _key(String tripId) => 'guest_$tripId';
+}