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/trip_document_service.dart

@@ -0,0 +1,49 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/trip_document.dart';
+
+/// Service that persists per-trip documents metadata locally.
+///
+/// Documents are stored as JSON in `SharedPreferences` to keep the UI
+/// responsive offline. Each trip key is `trip_docs_<tripId>`. The service is
+/// tolerant to corrupted payloads and resets gracefully to an empty list.
+class TripDocumentService {
+  /// Loads documents for the given [tripId]. Returns an empty list when none.
+  Future<List<TripDocument>> loadDocuments(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return TripDocument.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Saves the full document list for [tripId].
+  Future<void> saveDocuments(String tripId, List<TripDocument> docs) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), TripDocument.encodeList(docs));
+  }
+
+  /// Adds a new document entry and persists the updated list.
+  Future<List<TripDocument>> addDocument(
+    String tripId,
+    TripDocument doc,
+  ) async {
+    final current = await loadDocuments(tripId);
+    final updated = [...current, doc];
+    await saveDocuments(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes a document by [docId] and persists the change.
+  Future<List<TripDocument>> deleteDocument(String tripId, String docId) async {
+    final current = await loadDocuments(tripId);
+    final updated = current.where((d) => d.id != docId).toList();
+    await saveDocuments(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'trip_docs_$tripId';
+}