feat(portal-self-service): surface submit failures with a retryable alert (refs #68)

Add an error branch to submit(): on a failed BFF call, set a `failed` signal,
re-enable the button, and render a role="alert" message so the user knows the
submit did not go through and can retry — instead of the click silently doing
nothing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/self-service/src/app/registration/registration-page.html

@@ -8,6 +8,11 @@
       </p>
     } @else {
       <p utrecht-paragraph>U bent ingelogd met BSN {{ bsn() }}.</p>
+      @if (failed()) {
+        <p utrecht-paragraph role="alert">
+          Er ging iets mis bij het indienen van uw registratie. Probeer het opnieuw.
+        </p>
+      }
       <button
         utrecht-button
         appearance="primary-action-button"