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/emergency_contact.dart

@@ -0,0 +1,100 @@
+import 'dart:convert';
+
+/// Represents an emergency contact for a trip (person or service).
+///
+/// Stores basic contact details and optional notes for quick access during
+/// critical situations.
+class EmergencyContact {
+  /// Unique identifier for the contact entry.
+  final String id;
+
+  /// Display name (ex: "Ambassade", "Marie", "Assurance Europ").
+  final String name;
+
+  /// Phone number in international format when possible.
+  final String phone;
+
+  /// Optional description or role (ex: "Assistance médicale", "Famille").
+  final String? note;
+
+  /// Creation timestamp for stable ordering.
+  final DateTime createdAt;
+
+  /// Creates an emergency contact entry.
+  const EmergencyContact({
+    required this.id,
+    required this.name,
+    required this.phone,
+    required this.createdAt,
+    this.note,
+  });
+
+  /// Builds a new contact with current timestamp.
+  factory EmergencyContact.newContact({
+    required String id,
+    required String name,
+    required String phone,
+    String? note,
+  }) {
+    return EmergencyContact(
+      id: id,
+      name: name,
+      phone: phone,
+      note: note,
+      createdAt: DateTime.now().toUtc(),
+    );
+  }
+
+  /// Returns a copy with updated fields.
+  EmergencyContact copyWith({
+    String? id,
+    String? name,
+    String? phone,
+    String? note,
+    DateTime? createdAt,
+  }) {
+    return EmergencyContact(
+      id: id ?? this.id,
+      name: name ?? this.name,
+      phone: phone ?? this.phone,
+      note: note ?? this.note,
+      createdAt: createdAt ?? this.createdAt,
+    );
+  }
+
+  /// Serializes contact to JSON.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'name': name,
+      'phone': phone,
+      'note': note,
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+
+  /// Deserializes contact from JSON.
+  factory EmergencyContact.fromJson(Map<String, dynamic> json) {
+    return EmergencyContact(
+      id: json['id'] as String,
+      name: json['name'] as String,
+      phone: json['phone'] as String,
+      note: json['note'] as String?,
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  /// Encodes list to JSON string.
+  static String encodeList(List<EmergencyContact> contacts) {
+    return json.encode(contacts.map((c) => c.toJson()).toList());
+  }
+
+  /// Decodes list from JSON string.
+  static List<EmergencyContact> decodeList(String raw) {
+    final decoded = json.decode(raw) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(EmergencyContact.fromJson)
+        .toList();
+  }
+}