added product import and billing integration

lib/domain/repositories/transaction_repository.dart

@@ -13,6 +13,18 @@ abstract class TransactionRepository {
     Customer? updatedCustomer,
   });
 
+  /// Reverses a sale still inside its cancellation window: deletes the order,
+  /// restores the stock it consumed, and puts an attached shopper's loyalty
+  /// balance back to what it was immediately before the sale.
+  ///
+  /// Only valid before the bill has been offered to the back office — this
+  /// does not send a cancellation anywhere, it erases the sale as if it had
+  /// never happened locally.
+  Future<void> voidSale({
+    required SaleTransaction transaction,
+    required Map<String, double> stockMovements,
+  });
+
   Future<List<SaleTransaction>> history({int limit = 50});
 
   Future<SaleTransaction?> findByInvoice(String invoiceNumber);