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/models/packing_item.dart

@@ -0,0 +1,99 @@
+import 'dart:convert';
+
+/// Represents an item in the shared packing list for a trip.
+///
+/// Each item stores a [label], completion flag [isPacked], and an optional
+/// [assignee] to indicate who prend en charge l'item.
+class PackingItem {
+  /// Unique identifier for the packing entry.
+  final String id;
+
+  /// Text displayed in the list (ex: "Adaptateur US", "Pharmacie").
+  final String label;
+
+  /// Whether the item is already packed.
+  final bool isPacked;
+
+  /// Optional assignee (user id or name) for accountability.
+  final String? assignee;
+
+  /// Creation timestamp for ordering.
+  final DateTime createdAt;
+
+  /// Creates a packing item.
+  const PackingItem({
+    required this.id,
+    required this.label,
+    required this.isPacked,
+    required this.createdAt,
+    this.assignee,
+  });
+
+  /// Factory to create a new item in not-packed state.
+  factory PackingItem.newItem({
+    required String id,
+    required String label,
+    String? assignee,
+  }) {
+    return PackingItem(
+      id: id,
+      label: label,
+      assignee: assignee,
+      isPacked: false,
+      createdAt: DateTime.now().toUtc(),
+    );
+  }
+
+  /// Returns a copy with modifications.
+  PackingItem copyWith({
+    String? id,
+    String? label,
+    bool? isPacked,
+    String? assignee,
+    DateTime? createdAt,
+  }) {
+    return PackingItem(
+      id: id ?? this.id,
+      label: label ?? this.label,
+      isPacked: isPacked ?? this.isPacked,
+      assignee: assignee ?? this.assignee,
+      createdAt: createdAt ?? this.createdAt,
+    );
+  }
+
+  /// JSON serialization helper.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'label': label,
+      'isPacked': isPacked,
+      'assignee': assignee,
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+
+  /// JSON deserialization helper.
+  factory PackingItem.fromJson(Map<String, dynamic> json) {
+    return PackingItem(
+      id: json['id'] as String,
+      label: json['label'] as String,
+      isPacked: json['isPacked'] as bool? ?? false,
+      assignee: json['assignee'] as String?,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  /// Encodes list to JSON string.
+  static String encodeList(List<PackingItem> items) {
+    return json.encode(items.map((i) => i.toJson()).toList());
+  }
+
+  /// Decodes list from JSON string.
+  static List<PackingItem> decodeList(String raw) {
+    final decoded = json.decode(raw) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(PackingItem.fromJson)
+        .toList();
+  }
+}