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/emergency_service.dart

@@ -0,0 +1,55 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/emergency_contact.dart';
+
+/// Stores emergency contacts per trip for offline access.
+///
+/// Data is persisted locally in `SharedPreferences` under the key
+/// `emergency_<tripId>`. Corrupted payloads are cleaned up automatically to
+/// avoid crashing the UI during critical usage.
+class EmergencyService {
+  /// Loads contacts for [tripId]. Returns an empty list if none or corrupted.
+  Future<List<EmergencyContact>> loadContacts(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return EmergencyContact.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Saves the complete contact list.
+  Future<void> saveContacts(
+    String tripId,
+    List<EmergencyContact> contacts,
+  ) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), EmergencyContact.encodeList(contacts));
+  }
+
+  /// Adds a contact and returns updated list.
+  Future<List<EmergencyContact>> addContact(
+    String tripId,
+    EmergencyContact contact,
+  ) async {
+    final current = await loadContacts(tripId);
+    final updated = [...current, contact];
+    await saveContacts(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes a contact.
+  Future<List<EmergencyContact>> deleteContact(
+    String tripId,
+    String contactId,
+  ) async {
+    final current = await loadContacts(tripId);
+    final updated = current.where((c) => c.id != contactId).toList();
+    await saveContacts(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'emergency_$tripId';
+}