added product import and billing integration

lib/core/constants/app_constants.dart

@@ -39,8 +39,13 @@ class AppConstants {
   static const Duration barcodeScanTimeout = Duration(milliseconds: 120);
   static const int minBarcodeLength = 6;
 
-  /// Idle time after a completed sale before the terminal resets itself.
-  static const Duration postSaleResetDelay = Duration(seconds: 3);
+  /// Window after a completed sale during which the terminal waits, and the
+  /// bill is held back from the server — long enough for the cashier to
+  /// catch a mistake and cancel it before it becomes final. If the window
+  /// runs out (or "New Sale" is pressed early) the terminal resets and the
+  /// bill goes up; if it's cancelled first, the sale is voided and the cart
+  /// comes back exactly as it was.
+  static const Duration postSaleResetDelay = Duration(seconds: 30);
 
   static const int lowStockThreshold = 10;
   static const int maxParkedBills = 20;