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/packing_service.dart

@@ -0,0 +1,78 @@
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:travel_mate/models/packing_item.dart';
+
+/// Service handling shared packing lists per trip.
+///
+/// Uses local `SharedPreferences` for fast offline access. The list can later
+/// be synced remotely without changing the calling code.
+class PackingService {
+  /// Loads packing items for a trip. Returns empty list if none/corrupted.
+  Future<List<PackingItem>> loadItems(String tripId) async {
+    final prefs = await SharedPreferences.getInstance();
+    final raw = prefs.getString(_key(tripId));
+    if (raw == null) return const [];
+    try {
+      return PackingItem.decodeList(raw);
+    } catch (_) {
+      await prefs.remove(_key(tripId));
+      return const [];
+    }
+  }
+
+  /// Saves complete list.
+  Future<void> saveItems(String tripId, List<PackingItem> items) async {
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_key(tripId), PackingItem.encodeList(items));
+  }
+
+  /// Adds one item.
+  Future<List<PackingItem>> addItem(String tripId, PackingItem item) async {
+    final current = await loadItems(tripId);
+    final updated = [...current, item];
+    await saveItems(tripId, updated);
+    return updated;
+  }
+
+  /// Toggles packed flag.
+  Future<List<PackingItem>> toggleItem(String tripId, String itemId) async {
+    final current = await loadItems(tripId);
+    final updated = current
+        .map((i) {
+          if (i.id != itemId) return i;
+          return i.copyWith(isPacked: !i.isPacked);
+        })
+        .toList(growable: false);
+    await saveItems(tripId, updated);
+    return updated;
+  }
+
+  /// Deletes an item.
+  Future<List<PackingItem>> deleteItem(String tripId, String itemId) async {
+    final current = await loadItems(tripId);
+    final updated = current.where((i) => i.id != itemId).toList();
+    await saveItems(tripId, updated);
+    return updated;
+  }
+
+  /// Suggests a starter template based on duration/weather (simplified here).
+  List<String> suggestedItems({required int nights, required bool cold}) {
+    final base = [
+      'Passeport/ID',
+      'Billets / PNR',
+      'Chargeurs et adaptateurs',
+      'Trousse de secours',
+      'Assurance voyage',
+    ];
+    if (cold) {
+      base.addAll(['Veste chaude', 'Gants', 'Bonnet', 'Chaussettes chaudes']);
+    } else {
+      base.addAll(['Crème solaire', 'Lunettes de soleil', 'Maillot de bain']);
+    }
+    if (nights > 4) {
+      base.add('Lessive/ziplock');
+    }
+    return base;
+  }
+
+  String _key(String tripId) => 'packing_$tripId';
+}