(() => {
  const script = document.querySelector('script[data-customer-login-access]');
  if (!script) return;
  let pending = false;
  async function checkAccess() {
    if (pending || document.hidden) return;
    pending = true;
    try {
      const response = await fetch(script.dataset.customerLoginAccess, {
        credentials: 'same-origin', cache: 'no-store', headers: {Accept: 'application/json'}
      });
      if (!response.ok) return;
      const result = await response.json();
      if (!result.authenticated) window.location.replace(result.login_url);
    } catch (_) {
      // Retry on the next heartbeat or when the browser returns online.
    } finally {
      pending = false;
    }
  }
  window.setInterval(checkAccess, 10000);
  document.addEventListener('visibilitychange', checkAccess);
  window.addEventListener('pageshow', checkAccess);
  window.addEventListener('online', checkAccess);
})();