Show iframe when showing invoice in Shopify plugin

Close #4105

BTCPayServer/wwwroot/shopify/btcpay-shopify.js

@@ -133,6 +133,7 @@ window.BTCPayShopifyIntegrationModule = function () {
                 });
             });
             window.btcpay.showInvoice(currentInvoiceData.invoiceId);
+            window.btcpay.showFrame();
         }
     }