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/error_service.dart

@@ -1,18 +1,40 @@
 import 'package:flutter/material.dart';
 import '../components/error/error_content.dart';
 
+/// Service for handling application errors and user notifications.
+/// 
+/// This singleton service provides centralized error handling capabilities
+/// including displaying error dialogs, snackbars, and logging errors for
+/// debugging purposes. It uses a global navigator key to show notifications
+/// from anywhere in the application.
 class ErrorService {
   static final ErrorService _instance = ErrorService._internal();
+  
+  /// Factory constructor that returns the singleton instance.
   factory ErrorService() => _instance;
+  
+  /// Private constructor for singleton pattern.
   ErrorService._internal();
 
-  // GlobalKey pour accéder au context depuis n'importe où
+  /// Global navigator key for accessing context from anywhere in the app.
+  /// 
+  /// This key should be assigned to the MaterialApp's navigatorKey property
+  /// to enable error notifications from any part of the application.
   static GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
 
-  // Afficher une erreur en dialog
+  /// Displays an error message in a dialog.
+  /// 
+  /// Shows a modal dialog with the error message and optional retry functionality.
+  /// The dialog appearance can be customized with different icons and colors.
+  /// 
+  /// [message] - The error message to display
+  /// [title] - The dialog title (defaults to 'Error')
+  /// [onRetry] - Optional callback for retry functionality
+  /// [icon] - Icon to display in the dialog
+  /// [iconColor] - Color of the icon
   void showError({
     required String message,
-    String title = 'Erreur',
+    String title = 'Error',
     VoidCallback? onRetry,
     IconData icon = Icons.error_outline,
     Color? iconColor,
@@ -30,7 +52,14 @@ class ErrorService {
     }
   }
 
-  // Afficher une erreur en snackbar
+  /// Displays an error or success message in a snackbar.
+  /// 
+  /// Shows a floating snackbar at the bottom of the screen with the message.
+  /// The appearance changes based on whether it's an error or success message.
+  /// 
+  /// [message] - The message to display
+  /// [onRetry] - Optional callback for retry functionality
+  /// [isError] - Whether this is an error (true) or success (false) message
   void showSnackbar({
     required String message,
     VoidCallback? onRetry,
@@ -45,7 +74,7 @@ class ErrorService {
           duration: const Duration(seconds: 4),
           action: onRetry != null
               ? SnackBarAction(
-                  label: 'Réessayer',
+                  label: 'Retry',
                   textColor: Colors.white,
                   onPressed: onRetry,
                 )
@@ -59,10 +88,17 @@ class ErrorService {
     }
   }
 
-  // Logger dans la console (développement)
+  /// Logs error messages to the console during development.
+  /// 
+  /// Formats and displays error information including source, error message,
+  /// and optional stack trace in a visually distinct format.
+  /// 
+  /// [source] - The source or location where the error occurred
+  /// [error] - The error object or message
+  /// [stackTrace] - Optional stack trace for debugging
   void logError(String source, dynamic error, [StackTrace? stackTrace]) {
     print('═══════════════════════════════════');
-    print('❌ ERREUR dans $source');
+    print('❌ ERROR in $source');
     print('Message: $error');
     if (stackTrace != null) {
       print('StackTrace: $stackTrace');
@@ -70,12 +106,18 @@ class ErrorService {
     print('═══════════════════════════════════');
   }
 
-  // Logger une info (développement)
+  /// Logs informational messages to the console during development.
+  /// 
+  /// [source] - The source or location of the information
+  /// [message] - The informational message
   void logInfo(String source, String message) {
     print('ℹ️ [$source] $message');
   }
 
-  // Logger un succès
+  /// Logs success messages to the console during development.
+  /// 
+  /// [source] - The source or location of the success
+  /// [message] - The success message
   void logSuccess(String source, String message) {
     print('✅ [$source] $message');
   }