feat: Add User and UserBalance models with serialization methods

feat: Implement BalanceRepository for group balance calculations

feat: Create ExpenseRepository for managing expenses

feat: Add services for handling expenses and storage operations

fix: Update import paths for models in repositories and services

refactor: Rename CountContent to AccountContent in HomePage

chore: Add StorageService for image upload and management

lib/services/expense_service.dart

@@ -0,0 +1,91 @@
+import 'dart:io';
+import '../models/expense.dart';
+import '../repositories/expense_repository.dart';
+import 'error_service.dart';
+import 'storage_service.dart'; // Pour upload des reçus
+
+class ExpenseService {
+  final ExpenseRepository _expenseRepository;
+  final ErrorService _errorService;
+  final StorageService _storageService;
+
+  ExpenseService({
+    required ExpenseRepository expenseRepository,
+    ErrorService? errorService,
+    StorageService? storageService,
+  }) : _expenseRepository = expenseRepository,
+       _errorService = errorService ?? ErrorService(),
+       _storageService = storageService ?? StorageService();
+
+  // Création avec validation et upload d'image
+  Future<String> createExpenseWithValidation(Expense expense, File? receiptImage) async {
+    try {
+      // Validation métier
+      _validateExpenseData(expense);
+      
+      // Upload du reçu si présent
+      String? receiptUrl;
+      if (receiptImage != null) {
+        receiptUrl = await _storageService.uploadReceiptImage(
+          expense.groupId,
+          receiptImage,
+        );
+      }
+      
+      // Créer l'expense avec l'URL du reçu
+      final expenseWithReceipt = expense.copyWith(receiptUrl: receiptUrl);
+      
+      return await _expenseRepository.createExpense(expenseWithReceipt);
+    } catch (e) {
+      _errorService.logError('ExpenseService', 'Erreur création expense: $e');
+      rethrow;
+    }
+  }
+
+  // Mise à jour avec validation
+  Future<void> updateExpenseWithValidation(Expense expense, File? newReceiptImage) async {
+    try {
+      _validateExpenseData(expense);
+      
+      // Gérer le nouvel upload si nécessaire
+      String? receiptUrl = expense.receiptUrl;
+      if (newReceiptImage != null) {
+        receiptUrl = await _storageService.uploadReceiptImage(
+          expense.groupId,
+          newReceiptImage,
+        );
+      }
+      
+      final expenseWithReceipt = expense.copyWith(
+        receiptUrl: receiptUrl,
+        editedAt: DateTime.now(),
+        isEdited: true,
+      );
+      
+      await _expenseRepository.updateExpense(expenseWithReceipt);
+    } catch (e) {
+      _errorService.logError('ExpenseService', 'Erreur update expense: $e');
+      rethrow;
+    }
+  }
+
+  // Validation des données
+  void _validateExpenseData(Expense expense) {
+    if (expense.description.trim().isEmpty) {
+      throw Exception('La description est requise');
+    }
+
+    if (expense.amount <= 0) {
+      throw Exception('Le montant doit être positif');
+    }
+
+    if (expense.splits.isEmpty) {
+      throw Exception('Au moins un participant est requis');
+    }
+
+    final totalSplits = expense.splits.fold(0.0, (sum, split) => sum + split.amount);
+    if ((totalSplits - expense.amountInEur).abs() > 0.01) {
+      throw Exception('La somme des répartitions ne correspond pas au montant total');
+    }
+  }
+}