minor fixes

lib/interop.dart

@@ -1,12 +1,9 @@
 import 'dart:async';
 import 'dart:js_interop';
 
-import 'package:flutter_dotenv/flutter_dotenv.dart';
+import 'package:qpay/config/app_config.dart';
 import 'package:web/web.dart' as web;
 
-const String _defaultGatewayScriptUrl =
-    'https://test-gateway.mastercard.com/static/checkout/checkout.min.js';
-
 @JS('Checkout')
 external JSAny? _checkout;
 
@@ -23,8 +20,7 @@ Future<void> ensureCheckoutScriptLoaded() async {
     return;
   }
 
-  final scriptUrl =
-      dotenv.env['GATEWAY_SCRIPT_URL'] ?? _defaultGatewayScriptUrl;
+  final scriptUrl = AppConfig.gatewayScriptUrl;
 
   final scriptElement = web.HTMLScriptElement()
     ..id = 'mastercard-checkout-script'