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/models/user.dart

@@ -1,11 +1,27 @@
 import 'dart:convert';
 
+/// Model representing a user in the travel mate application.
+/// 
+/// This class encapsulates user information including personal details
+/// and provides methods for serialization/deserialization with Firebase
+/// and JSON operations.
 class User {
+  /// Unique identifier for the user (usually Firebase UID).
   final String? id;
+  
+  /// User's last name.
   final String nom;
+  
+  /// User's first name.
   final String prenom;
+  
+  /// User's email address.
   final String email;
 
+  /// Creates a new [User] instance.
+  /// 
+  /// [nom], [prenom], and [email] are required fields.
+  /// [id] is optional and typically assigned by Firebase.
   User({
     this.id,
     required this.nom,
@@ -13,7 +29,9 @@ class User {
     required this.email,
   });
 
-  // Constructeur pour créer un User depuis un Map (utile pour Firebase)
+  /// Creates a [User] instance from a Map (useful for Firebase operations).
+  /// 
+  /// Handles null values gracefully by providing empty string defaults.
   factory User.fromMap(Map<String, dynamic> map) {
     return User(
       id: map['id'],
@@ -23,13 +41,17 @@ class User {
     );
   }
 
-  // Constructeur pour créer un User depuis JSON
+  /// Creates a [User] instance from a JSON string.
+  /// 
+  /// Parses the JSON and delegates to [fromMap] for object creation.
   factory User.fromJson(String jsonStr) {
     Map<String, dynamic> map = json.decode(jsonStr);
     return User.fromMap(map);
   }
 
-  // Méthode pour convertir un User en Map (utile pour Firebase)
+  /// Converts the [User] instance to a Map (useful for Firebase operations).
+  /// 
+  /// Returns a map with all user properties for database storage.
   Map<String, dynamic> toMap() {
     return {
       'id': id,
@@ -39,15 +61,20 @@ class User {
     };
   }
 
-  // Méthode pour convertir un User en JSON
+  /// Converts the [User] instance to a JSON string.
+  /// 
+  /// Useful for API communications and data serialization.
   String toJson() {
     return json.encode(toMap());
   }
 
-  // Méthode pour obtenir le nom complet
+  /// Gets the user's full name in "first last" format.
   String get fullName => '$prenom $nom';
 
-  // Méthode pour créer une copie avec des modifications
+  /// Creates a copy of this user with optionally modified properties.
+  /// 
+  /// Allows updating specific fields while preserving others.
+  /// Useful for state management and partial updates.
   User copyWith({
     String? id,
     String? nom,
@@ -62,17 +89,22 @@ class User {
     );
   }
 
+  /// Returns a string representation of the user.
   @override
   String toString() {
     return 'User(id: $id, nom: $nom, prenom: $prenom, email: $email)';
   }
 
+  /// Compares users based on email address.
+  /// 
+  /// Two users are considered equal if they have the same email.
   @override
   bool operator ==(Object other) {
     if (identical(this, other)) return true;
     return other is User && other.email == email;
   }
 
+  /// Hash code based on email address.
   @override
   int get hashCode => email.hashCode;
 }