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/auth_repository.dart

@@ -3,23 +3,49 @@ import 'package:cloud_firestore/cloud_firestore.dart';
 import '../models/user.dart';
 import '../services/auth_service.dart';
 
+/// Repository for authentication operations and user data management.
+/// 
+/// This repository handles authentication logic and user data persistence
+/// by coordinating between Firebase Auth and Firestore. It provides a
+/// clean interface for authentication operations while managing the
+/// relationship between Firebase users and application user data.
 class AuthRepository {
+  /// Authentication service for Firebase Auth operations.
   final AuthService _authService;
+  
+  /// Firestore instance for user data operations.
   final FirebaseFirestore _firestore;
 
+  /// Creates a new [AuthRepository] with optional service dependencies.
+  /// 
+  /// If [authService] or [firestore] are not provided, default instances will be used.
   AuthRepository({
     AuthService? authService,
     FirebaseFirestore? firestore,
   })  : _authService = authService ?? AuthService(),
         _firestore = firestore ?? FirebaseFirestore.instance;
 
-  // Vérifier l'état de connexion actuel
+  /// Stream of authentication state changes.
+  /// 
+  /// Emits Firebase user objects when authentication state changes.
   Stream<firebase_auth.User?> get authStateChanges =>
       _authService.authStateChanges;
 
+  /// Gets the currently authenticated Firebase user.
+  /// 
+  /// Returns null if no user is currently authenticated.
   firebase_auth.User? get currentUser => _authService.currentUser;
 
-  // Connexion avec email/mot de passe
+  /// Signs in a user with email and password.
+  /// 
+  /// Authenticates with Firebase Auth and retrieves the corresponding
+  /// user data from Firestore.
+  /// 
+  /// [email] - User's email address
+  /// [password] - User's password
+  /// 
+  /// Returns the [User] model if successful, null if user data not found.
+  /// Throws an exception if authentication fails.
   Future<User?> signInWithEmailAndPassword({
     required String email,
     required String password,
@@ -31,11 +57,22 @@ class AuthRepository {
       );
       return await getUserFromFirestore(firebaseUser.user!.uid);
     } catch (e) {
-      throw Exception('Erreur de connexion: $e');
+      throw Exception('Sign-in error: $e');
     }
   }
 
-  // Inscription avec email/mot de passe
+  /// Creates a new user account with email and password.
+  /// 
+  /// Creates a Firebase Auth account and stores additional user information
+  /// in Firestore.
+  /// 
+  /// [email] - User's email address
+  /// [password] - User's password
+  /// [nom] - User's last name
+  /// [prenom] - User's first name
+  /// 
+  /// Returns the created [User] model if successful.
+  /// Throws an exception if account creation fails.
   Future<User?> signUpWithEmailAndPassword({
     required String email,
     required String password,
@@ -48,7 +85,7 @@ class AuthRepository {
         password: password,
       );
 
-      // Créer le document utilisateur dans Firestore
+      // Create user document in Firestore with additional information
       final user = User(
         id: firebaseUser.user!.uid,
         email: email,
@@ -60,29 +97,35 @@ class AuthRepository {
       return user;
       
     } catch (e) {
-      throw Exception('Erreur d\'inscription: $e');
+      throw Exception('Registration error: $e');
     }
   }
 
-  // Connexion avec Google
+  /// Signs in a user using Google authentication.
+  /// 
+  /// Handles Google sign-in flow and creates/retrieves user data from Firestore.
+  /// If the user doesn't exist, creates a new user document.
+  /// 
+  /// Returns the [User] model if successful, null if Google sign-in was cancelled.
+  /// Throws an exception if authentication fails.
   Future<User?> signInWithGoogle() async {
     try {
       final firebaseUser = await _authService.signInWithGoogle();
 
       if (firebaseUser.user != null) {
-        // Vérifier si l'utilisateur existe déjà
+        // Check if user already exists in Firestore
         final existingUser = await getUserFromFirestore(firebaseUser.user!.uid);
         
         if (existingUser != null) {
           return existingUser;
         }
 
-        // Créer un nouvel utilisateur
+        // Create new user document for first-time Google sign-in
         final user = User(
           id: firebaseUser.user!.uid,
           email: firebaseUser.user!.email ?? '',
           nom: '',
-          prenom: firebaseUser.user!.displayName ?? 'Utilisateur',
+          prenom: firebaseUser.user!.displayName ?? 'User',
         );
 
         await _firestore.collection('users').doc(user.id).set(user.toMap());
@@ -90,11 +133,17 @@ class AuthRepository {
       }
       return null;
     } catch (e) {
-      throw Exception('Erreur de connexion Google: $e');
+      throw Exception('Google sign-in error: $e');
     }
   }
 
-  // Connexion avec Apple
+  /// Signs in a user using Apple authentication.
+  /// 
+  /// Handles Apple sign-in flow and creates/retrieves user data from Firestore.
+  /// If the user doesn't exist, creates a new user document.
+  /// 
+  /// Returns the [User] model if successful, null if Apple sign-in was cancelled.
+  /// Throws an exception if authentication fails.
   Future<User?> signInWithApple() async {
     try {
       final firebaseUser = await _authService.signInWithApple();
@@ -110,7 +159,7 @@ class AuthRepository {
           id: firebaseUser.user!.uid,
           email: firebaseUser.user!.email ?? '',
           nom: '',
-          prenom: firebaseUser.user!.displayName ?? 'Utilisateur',
+          prenom: firebaseUser.user!.displayName ?? 'User',
         );
 
         await _firestore.collection('users').doc(user.id).set(user.toMap());
@@ -118,21 +167,34 @@ class AuthRepository {
       }
       return null;
     } catch (e) {
-      throw Exception('Erreur de connexion Apple: $e');
+      throw Exception('Apple sign-in error: $e');
     }
   }
 
-  // Déconnexion
+  /// Signs out the current user.
+  /// 
+  /// Clears the authentication state and signs out from Firebase Auth.
   Future<void> signOut() async {
     await _authService.signOut();
   }
 
-  // Réinitialisation du mot de passe
+  /// Sends a password reset email to the specified email address.
+  /// 
+  /// [email] - The email address to send the reset link to
+  /// 
+  /// Throws an exception if the operation fails.
   Future<void> resetPassword(String email) async {
     await _authService.resetPassword(email);
   }
 
-  // Récupérer les données utilisateur depuis Firestore
+  /// Retrieves user data from Firestore by user ID.
+  /// 
+  /// Fetches the user document from the 'users' collection and converts
+  /// it to a [User] model.
+  /// 
+  /// [uid] - The Firebase user ID to look up
+  /// 
+  /// Returns the [User] model if found, null otherwise.
   Future<User?> getUserFromFirestore(String uid) async {
     try {
       final doc = await _firestore.collection('users').doc(uid).get();