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/repositories/user_repository.dart

@@ -2,17 +2,35 @@ import 'package:cloud_firestore/cloud_firestore.dart';
 import '../models/user.dart';
 import '../services/auth_service.dart';
 
+/// Repository for user data operations in Firestore.
+/// 
+/// This repository provides methods for CRUD operations on user data,
+/// including retrieving users by ID or email, updating user information,
+/// and managing user profiles in the Firestore database.
 class UserRepository {
+  /// Firestore instance for database operations.
   final FirebaseFirestore _firestore;
+  
+  /// Authentication service for user-related operations.
   final AuthService _authService;
 
+  /// Creates a new [UserRepository] with optional dependencies.
+  /// 
+  /// If [firestore] or [authService] are not provided, default instances will be used.
   UserRepository({
     FirebaseFirestore? firestore,
     AuthService? authService,
   })  : _firestore = firestore ?? FirebaseFirestore.instance,
         _authService = authService ?? AuthService();
 
-  // Récupérer un utilisateur par ID
+  /// Retrieves a user by their unique ID.
+  /// 
+  /// Fetches the user document from Firestore and converts it to a [User] model.
+  /// 
+  /// [uid] - The unique user identifier
+  /// 
+  /// Returns the [User] model if found, null otherwise.
+  /// Throws an exception if the operation fails.
   Future<User?> getUserById(String uid) async {
     try {
       final doc = await _firestore.collection('users').doc(uid).get();
@@ -22,11 +40,19 @@ class UserRepository {
       }
       return null;
     } catch (e) {
-      throw Exception('Erreur lors de la récupération de l\'utilisateur: $e');
+      throw Exception('Error retrieving user: $e');
     }
   }
 
-  // Récupérer un utilisateur par email
+  /// Retrieves a user by their email address.
+  /// 
+  /// Searches the users collection for a matching email address.
+  /// Email comparison is case-insensitive after trimming whitespace.
+  /// 
+  /// [email] - The email address to search for
+  /// 
+  /// Returns the first [User] found with the matching email, null if not found.
+  /// Throws an exception if the operation fails.
   Future<User?> getUserByEmail(String email) async {
     try {
       final querySnapshot = await _firestore
@@ -42,11 +68,17 @@ class UserRepository {
       }
       return null;
     } catch (e) {
-      throw Exception('Erreur lors de la recherche de l\'utilisateur: $e');
+      throw Exception('Error searching for user: $e');
     }
   }
 
-  // Mettre à jour un utilisateur
+  /// Updates an existing user in Firestore.
+  /// 
+  /// Updates the user document with the provided user data.
+  /// 
+  /// [user] - The user object containing updated information
+  /// 
+  /// Returns true if the update was successful, false otherwise.
   Future<bool> updateUser(User user) async {
     try {
       await _firestore.collection('users').doc(user.id).update(user.toMap());