Improve Gateway authentication and static HTTP fidelity

ui/login.js

@@ -2,19 +2,31 @@
 const form = document.getElementById('login-form');
 const button = document.getElementById('nsg-x1-logon-button');
 const message = document.getElementById('message');
+async function authenticate(path, body) {
+  const response = await fetch(path, {
+    method: 'POST', body, credentials: 'same-origin', redirect: 'error',
+    headers: { Accept: 'application/vnd.citrix.authenticateresponse-1+xml' },
+    signal: AbortSignal.timeout(10000)
+  });
+  if (!response.ok) throw new Error('Request failed');
+  const result = rdx.utils.authentication(await response.text());
+  form.elements.StateContext.value = result.state;
+  // Only the local authentication endpoint can receive form credentials.
+  if (result.postback === '/nf/auth/doAuthentication.do') form.action = result.postback;
+  return result;
+}
+const ready = authenticate('/nf/auth/getAuthenticationRequirements.do', new URLSearchParams())
+  .catch(() => null);
 form.addEventListener('submit', async event => {
   event.preventDefault();
   button.disabled = true;
   message.textContent = '';
   try {
+    await ready;
     const body = new URLSearchParams(new FormData(form));
     body.set('nsg-x1-logon-button', 'Log On');
-    const response = await fetch('/nf/auth/doAuthentication.do', {
-      method: 'POST', body, credentials: 'same-origin', redirect: 'error',
-      signal: AbortSignal.timeout(10000)
-    });
-    if (!response.ok) throw new Error('Request failed');
-    message.textContent = 'Anmeldung fehlgeschlagen. Überprüfen Sie Ihre Zugangsdaten.';
+    const result = await authenticate('/nf/auth/doAuthentication.do', body);
+    message.textContent = result.errors.join(' ') || 'Anmeldung fehlgeschlagen. Überprüfen Sie Ihre Zugangsdaten.';
   } catch {
     message.textContent = 'Die Anmeldung ist derzeit nicht möglich. Versuchen Sie es erneut.';
   } finally {