feat: Implement group balance and expense management with new navigation and data handling

lib/repositories/balance_repository.dart

@@ -43,6 +43,19 @@ class BalanceRepository {
     }
   }
 
+  Future<List<UserBalance>> calculateGroupUserBalances(String groupId) async {
+    try {
+      final expenses = await _expenseRepository
+          .getExpensesStream(groupId)
+          .first;
+
+      return _calculateUserBalances(expenses);
+    } catch (e) {
+      _errorService.logError('BalanceRepository', 'Erreur calcul user balances: $e');
+      rethrow;
+    }
+  }
+
   // Calculer les balances individuelles
   List<UserBalance> _calculateUserBalances(List<Expense> expenses) {
     final Map<String, Map<String, dynamic>> userBalanceMap = {};