app.html

<!DOCTYPE html>
<html {{ HTML_ATTRS }}>

<head>
  {{ HEAD }}
  <script defer src="https://identity.netlify.com/v1/netlify-identity-widget.js"></script>
</head>

<body {{ BODY_ATTRS }}>
  {{ APP }}
</body>
<script>
  if (window.netlifyIdentity) {
    window.netlifyIdentity.on("init", function (user) {
      if (!user) {
        window.netlifyIdentity.on("login", function () {
          document.location.href = "/admin/";
        });
      }
    });
  }
</script>

</html>