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/blocs/user/user_state.dart

@@ -1,39 +1,81 @@
 import 'package:equatable/equatable.dart';
 
+/// Abstract base class for all user-related states.
+/// 
+/// This class extends [Equatable] to enable value equality for state comparison.
+/// All user states in the application should inherit from this class.
 abstract class UserState extends Equatable {
   @override
   List<Object?> get props => [];
 }
 
+/// Initial state of the user bloc.
+/// 
+/// This state represents the initial state before any user operations
+/// have been performed or when the user has logged out.
 class UserInitial extends UserState {}
 
+/// State indicating that a user operation is in progress.
+/// 
+/// This state is used to show loading indicators during user data
+/// operations like loading, updating, or initializing user information.
 class UserLoading extends UserState {}
 
+/// State indicating that user data has been successfully loaded.
+/// 
+/// This state contains the loaded user information and is used
+/// throughout the app to access current user data.
 class UserLoaded extends UserState {
+  /// The loaded user data.
   final UserModel user;
   
+  /// Creates a [UserLoaded] state with the given [user] data.
   UserLoaded(this.user);
   
   @override
   List<Object?> get props => [user];
 }
 
+/// State indicating that a user operation has failed.
+/// 
+/// This state contains an error message that can be displayed to the user
+/// when user operations fail.
 class UserError extends UserState {
+  /// The error message describing what went wrong.
   final String message;
   
+  /// Creates a [UserError] state with the given error [message].
   UserError(this.message);
   
   @override
   List<Object?> get props => [message];
 }
 
-// Modèle utilisateur simple
+/// Simple user model for representing user data in the application.
+/// 
+/// This model contains basic user information and provides methods for
+/// serialization/deserialization with Firestore operations.
+/// Simple user model for representing user data in the application.
+/// 
+/// This model contains basic user information and provides methods for
+/// serialization/deserialization with Firestore operations.
 class UserModel {
+  /// Unique identifier for the user (Firebase UID).
   final String id;
+  
+  /// User's email address.
   final String email;
+  
+  /// User's first name.
   final String prenom;
+  
+  /// User's last name (optional).
   final String? nom;
   
+  /// Creates a new [UserModel] instance.
+  /// 
+  /// [id], [email], and [prenom] are required fields.
+  /// [nom] is optional and can be null.
   UserModel({
     required this.id,
     required this.email,
@@ -41,6 +83,10 @@ class UserModel {
     this.nom,
   });
   
+  /// Creates a [UserModel] instance from a JSON map.
+  /// 
+  /// Handles null values gracefully by providing default values.
+  /// [prenom] defaults to 'Voyageur' (Traveler) if not provided.
   factory UserModel.fromJson(Map<String, dynamic> json) {
     return UserModel(
       id: json['id'] ?? '',
@@ -50,6 +96,9 @@ class UserModel {
     );
   }
   
+  /// Converts the [UserModel] instance to a JSON map.
+  /// 
+  /// Useful for storing user data in Firestore or other JSON-based operations.
   Map<String, dynamic> toJson() {
     return {
       'id': id,