improved environment management

lib/interop.dart

@@ -1,7 +1,48 @@
+import 'dart:async';
 import 'dart:js_interop';
 
+import 'package:flutter_dotenv/flutter_dotenv.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;
+
 @JS('configure')
 external void configure(String sessionID);
 
 @JS('showEmbeddedPage')
 external void showEmbeddedPage();
+
+Future<void> ensureCheckoutScriptLoaded() async {
+  final existing = web.document.querySelector('#mastercard-checkout-script');
+  if (existing is web.HTMLScriptElement) {
+    await _waitForCheckout();
+    return;
+  }
+
+  final scriptUrl =
+      dotenv.env['GATEWAY_SCRIPT_URL'] ?? _defaultGatewayScriptUrl;
+
+  final scriptElement = web.HTMLScriptElement()
+    ..id = 'mastercard-checkout-script'
+    ..src = scriptUrl
+    ..async = true;
+
+  web.document.head?.append(scriptElement);
+  await _waitForCheckout();
+}
+
+Future<void> _waitForCheckout() async {
+  const maxAttempts = 100;
+  for (var attempt = 0; attempt < maxAttempts; attempt++) {
+    if (_checkout != null) {
+      return;
+    }
+    await Future.delayed(const Duration(milliseconds: 100));
+  }
+
+  throw StateError('Mastercard Checkout script failed to load before timeout.');
+}