Enhance model and service documentation with detailed comments and descriptions

- Updated Group, Trip, User, and other model classes to include comprehensive documentation for better understanding and maintainability.
- Improved error handling and logging in services, including AuthService, ErrorService, and StorageService.
- Added validation and business logic explanations in ExpenseService and TripService.
- Refactored method comments to follow a consistent format across the codebase.
- Translated error messages and comments from French to English for consistency.

lib/services/user_service.dart

@@ -3,29 +3,53 @@ import 'package:firebase_auth/firebase_auth.dart';
 import 'package:travel_mate/services/error_service.dart';
 import '../blocs/user/user_state.dart';
 
+/// Service for managing user operations with Firestore and Firebase Auth.
+/// 
+/// This service provides functionality for user management including creating,
+/// retrieving, updating, and deleting user data in Firestore. It also handles
+/// user authentication state and provides methods for user profile management.
 class UserService {
+  /// Error service for logging user operation errors.
   final _errorService = ErrorService();
+  
+  /// Firestore instance for database operations.
   final FirebaseFirestore _firestore;
+  
+  /// Firebase Auth instance for user authentication.
   final FirebaseAuth _auth;
+  
+  /// Collection name for users in Firestore.
   static const String _usersCollection = 'users';
 
+  /// Creates a new [UserService] with optional Firestore and Auth instances.
+  /// 
+  /// If [firestore] or [auth] are not provided, the default instances will be used.
   UserService({
     FirebaseFirestore? firestore,
     FirebaseAuth? auth,
   })  : _firestore = firestore ?? FirebaseFirestore.instance,
         _auth = auth ?? FirebaseAuth.instance;
 
-  // Obtenir l'utilisateur connecté actuel
+  /// Gets the currently authenticated Firebase user.
+  /// 
+  /// Returns the [User] object if authenticated, null otherwise.
   User? getCurrentFirebaseUser() {
     return _auth.currentUser;
   }
 
-  // Obtenir l'ID de l'utilisateur connecté
+  /// Gets the ID of the currently authenticated user.
+  /// 
+  /// Returns the user ID string if authenticated, null otherwise.
   String? getCurrentUserId() {
     return _auth.currentUser?.uid;
   }
 
-  // Créer un nouvel utilisateur dans Firestore
+  /// Creates a new user document in Firestore.
+  /// 
+  /// Takes a [UserModel] object and stores it in the users collection.
+  /// Returns true if successful, false if an error occurs.
+  /// 
+  /// [user] - The user model to create in Firestore
   Future<bool> createUser(UserModel user) async {
     try {
       await _firestore
@@ -34,12 +58,16 @@ class UserService {
           .set(user.toJson());
       return true;
     } catch (e) {
-      _errorService.logError('Erreur lors de la création de l\'utilisateur: $e', StackTrace.current);
+      _errorService.logError('Error creating user: $e', StackTrace.current);
       return false;
     }
   }
 
-  // Obtenir un utilisateur par son ID
+  /// Retrieves a user by their ID from Firestore.
+  /// 
+  /// Returns a [UserModel] if the user exists, null otherwise.
+  /// 
+  /// [userId] - The ID of the user to retrieve
   Future<UserModel?> getUserById(String userId) async {
     try {
       final doc = await _firestore