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/album_service.dart

@@ -0,0 +1,42 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/album_photo.dart';
+
+/// Stores shared album photos per trip locally for offline access.
+class AlbumService {
+  /// Loads photos for the given trip.
+  Future<List<AlbumPhoto>> loadPhotos(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return AlbumPhoto.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Saves photo list.
+  Future<void> savePhotos(String tripId, List<AlbumPhoto> photos) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), AlbumPhoto.encodeList(photos));
+  }
+
+  /// Adds one photo.
+  Future<List<AlbumPhoto>> addPhoto(String tripId, AlbumPhoto photo) async {
+    final current = await loadPhotos(tripId);
+    final updated = [...current, photo];
+    await savePhotos(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes a photo.
+  Future<List<AlbumPhoto>> deletePhoto(String tripId, String photoId) async {
+    final current = await loadPhotos(tripId);
+    final updated = current.where((p) => p.id != photoId).toList();
+    await savePhotos(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'album_$tripId';
+}