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

lib/services/balance_service.dart

@@ -30,6 +30,20 @@ class BalanceService {
     }
   }
 
+  Future<List<Settlement>> calculateOptimalSettlements(String groupId) async {
+    try {
+      final expenses = await _expenseRepository
+          .getExpensesStream(groupId)
+          .first;
+
+      final userBalances = calculateUserBalances(expenses);
+      return optimizeSettlements(userBalances);
+    } catch (e) {
+      _errorService.logError('BalanceService', 'Erreur calcul settlements: $e');
+      rethrow;
+    }
+  }
+
   /// Stream de la balance en temps réel
   Stream<GroupBalance> getGroupBalanceStream(String groupId) {
     return _expenseRepository.getExpensesStream(groupId).asyncMap((expenses) async {