add prompt confirmation and update carton

lib/domain/entities/discount.dart

@@ -35,4 +35,10 @@ class Discount {
       status: map['status'],
     );
   }
+
+  bool isChangedForEdit(Discount discount) {
+    return discount.code != this.code ||
+        discount.amount != this.amount ||
+        discount.customerId != this.customerId;
+  }
 }