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/budget_category.dart

@@ -0,0 +1,106 @@
+import 'dart:convert';
+
+/// Represents a budget envelope per category with currency awareness.
+class BudgetCategory {
+  /// Unique identifier of the category entry.
+  final String id;
+
+  /// Category name (hébergement, transport, food, activités...).
+  final String name;
+
+  /// Planned amount.
+  final double planned;
+
+  /// Currency code (ISO 4217) used for the amount.
+  final String currency;
+
+  /// Amount actually spent (to be filled by expenses sync later).
+  final double spent;
+
+  /// Creation timestamp for ordering.
+  final DateTime createdAt;
+
+  /// Creates a budget category entry.
+  const BudgetCategory({
+    required this.id,
+    required this.name,
+    required this.planned,
+    required this.currency,
+    required this.spent,
+    required this.createdAt,
+  });
+
+  /// Convenience constructor for new envelope.
+  factory BudgetCategory.newCategory({
+    required String id,
+    required String name,
+    required double planned,
+    required String currency,
+  }) {
+    return BudgetCategory(
+      id: id,
+      name: name,
+      planned: planned,
+      currency: currency,
+      spent: 0,
+      createdAt: DateTime.now().toUtc(),
+    );
+  }
+
+  /// Returns a copy with updated fields.
+  BudgetCategory copyWith({
+    String? id,
+    String? name,
+    double? planned,
+    String? currency,
+    double? spent,
+    DateTime? createdAt,
+  }) {
+    return BudgetCategory(
+      id: id ?? this.id,
+      name: name ?? this.name,
+      planned: planned ?? this.planned,
+      currency: currency ?? this.currency,
+      spent: spent ?? this.spent,
+      createdAt: createdAt ?? this.createdAt,
+    );
+  }
+
+  /// JSON serialization.
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'name': name,
+      'planned': planned,
+      'currency': currency,
+      'spent': spent,
+      'createdAt': createdAt.toIso8601String(),
+    };
+  }
+
+  /// JSON deserialization.
+  factory BudgetCategory.fromJson(Map<String, dynamic> json) {
+    return BudgetCategory(
+      id: json['id'] as String,
+      name: json['name'] as String,
+      planned: (json['planned'] as num).toDouble(),
+      currency: json['currency'] as String,
+      spent: (json['spent'] as num).toDouble(),
+      createdAt: DateTime.parse(json['createdAt'] as String),
+    );
+  }
+
+  /// Encodes a list to JSON string.
+  static String encodeList(List<BudgetCategory> categories) {
+    return json.encode(categories.map((c) => c.toJson()).toList());
+  }
+
+  /// Decodes a list from JSON string.
+  static List<BudgetCategory> decodeList(String raw) {
+    final decoded = json.decode(raw) as List<dynamic>;
+    return decoded
+        .cast<Map<String, dynamic>>()
+        .map(BudgetCategory.fromJson)
+        .toList();
+  }
+}