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/expense_service.dart

@@ -2,13 +2,27 @@ import 'dart:io';
 import '../models/expense.dart';
 import '../repositories/expense_repository.dart';
 import 'error_service.dart';
-import 'storage_service.dart'; // Pour upload des reçus
+import 'storage_service.dart';
 
+/// Service for managing expense operations with business logic validation.
+/// 
+/// This service provides high-level expense management functionality including
+/// validation, receipt image uploading, and coordination with the expense repository.
+/// It acts as a business logic layer between the UI and data persistence.
 class ExpenseService {
+  /// Repository for expense data operations.
   final ExpenseRepository _expenseRepository;
+  
+  /// Service for error handling and logging.
   final ErrorService _errorService;
+  
+  /// Service for handling file uploads (receipts).
   final StorageService _storageService;
 
+  /// Creates a new [ExpenseService] with required dependencies.
+  /// 
+  /// [expenseRepository] is required for data operations.
+  /// [errorService] and [storageService] have default implementations if not provided.
   ExpenseService({
     required ExpenseRepository expenseRepository,
     ErrorService? errorService,
@@ -17,13 +31,22 @@ class ExpenseService {
        _errorService = errorService ?? ErrorService(),
        _storageService = storageService ?? StorageService();
 
-  // Création avec validation et upload d'image
+  /// Creates an expense with validation and optional receipt image upload.
+  /// 
+  /// Validates the expense data, uploads receipt image if provided, and
+  /// creates the expense record in the database.
+  /// 
+  /// [expense] - The expense data to create
+  /// [receiptImage] - Optional receipt image file to upload
+  /// 
+  /// Returns the ID of the created expense.
+  /// Throws exceptions if validation fails or creation errors occur.
   Future<String> createExpenseWithValidation(Expense expense, File? receiptImage) async {
     try {
-      // Validation métier
+      // Business logic validation
       _validateExpenseData(expense);
       
-      // Upload du reçu si présent
+      // Upload receipt image if provided
       String? receiptUrl;
       if (receiptImage != null) {
         receiptUrl = await _storageService.uploadReceiptImage(