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/components/error/error_content.dart

@@ -1,16 +1,35 @@
 import 'package:flutter/material.dart';
 
+/// A reusable error display component.
+/// 
+/// This widget provides a consistent way to display error messages throughout
+/// the application. It supports customizable titles, messages, icons, and
+/// action buttons for retry and close operations.
 class ErrorContent extends StatelessWidget {
+  /// The error title to display
   final String title;
+  
+  /// The error message to display
   final String message;
+  
+  /// Optional callback for retry action
   final VoidCallback? onRetry;
+  
+  /// Optional callback for close action
   final VoidCallback? onClose;
+  
+  /// Icon to display with the error
   final IconData icon;
+  
+  /// Color of the error icon
   final Color? iconColor;
 
+  /// Creates a new [ErrorContent] widget.
+  /// 
+  /// [message] is required, other parameters are optional with sensible defaults.
   const ErrorContent({
     super.key,
-    this.title = 'Une erreur est survenue',
+    this.title = 'An error occurred',
     required this.message,
     this.onRetry,
     this.onClose,