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_checklist_service.dart

@@ -0,0 +1,77 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/checklist_item.dart';
+
+/// Service responsible for storing and retrieving per-trip checklists.
+///
+/// Persistence relies on `SharedPreferences` with one JSON string per trip
+/// key. All methods are resilient to corrupted payloads and return empty
+/// lists rather than throwing to keep the UI responsive.
+class TripChecklistService {
+  /// Loads the checklist items for the given [tripId].
+  ///
+  /// Returns an empty list if no data exists or if deserialization fails.
+  Future<List<ChecklistItem>> loadChecklist(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+
+    if (raw == null) {
+      return const [];
+    }
+
+    try {
+      return ChecklistItem.decodeList(raw);
+    } catch (_) {
+      // Corrupted payload: clear it to avoid persisting errors.
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Persists the provided [items] list for [tripId].
+  ///
+  /// This method overrides the previously stored list; use helpers like
+  /// [addItem], [toggleItem], or [deleteItem] for incremental updates.
+  Future<void> saveChecklist(String tripId, List<ChecklistItem> items) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), ChecklistItem.encodeList(items));
+  }
+
+  /// Adds a new [item] to the checklist for [tripId].
+  ///
+  /// Items are appended in creation order; the updated list is persisted.
+  Future<List<ChecklistItem>> addItem(String tripId, ChecklistItem item) async {
+    final current = await loadChecklist(tripId);
+    final updated = [...current, item];
+    await saveChecklist(tripId, updated);
+    return updated;
+  }
+
+  /// Toggles the completion state of the item matching [itemId].
+  ///
+  /// Returns the updated list. If the item is not found, the list is
+  /// returned unchanged to keep UI state consistent.
+  Future<List<ChecklistItem>> toggleItem(String tripId, String itemId) async {
+    final current = await loadChecklist(tripId);
+    final updated = current
+        .map((item) {
+          if (item.id != itemId) {
+            return item;
+          }
+          return item.copyWith(isDone: !item.isDone);
+        })
+        .toList(growable: false);
+
+    await saveChecklist(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes the item matching [itemId] and persists the change.
+  Future<List<ChecklistItem>> deleteItem(String tripId, String itemId) async {
+    final current = await loadChecklist(tripId);
+    final updated = current.where((item) => item.id != itemId).toList();
+    await saveChecklist(tripId, updated);
+    return updated;
+  }
+
+  String _key(String tripId) => 'checklist_$tripId';
+}