server/public/index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>entropychat.app</title>
    <meta name="description" content="entropychat.app">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }

      html, body {
        font-family: 'Helvetica Neue', 'Helvetica', 'Arial', 'sans-serif';
        font-weight: 400;
        font-size: 16px;
        background: #290016;
        color: #dc136c;
        width: 100vw;
        height: 100vh;
        display: flex;
        justify-content: center;
        align-items: center;
        flex-direction: column;
      }

      h1 { margin: 1em 0 0 0; font-weight: 300; }
    </style>
  </head>
  <body>
    <img src="https://i.imgur.com/KQDKLRa.png">
    <h1 id="info">Logging you in...</h1>
    <script>
      const params = new URLSearchParams(window.location.hash.slice(1));
      const token = params.get('access_token');
      if (window.opener && token) {
        // TODO: dynamically set origin
        window.opener.postMessage({ token }, 'http://localhost:8080');
      } else {
        info.textContent = 'Go home.';
      }
      window.close();
    </script>
  </body>
</html>