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"

apps/self-service/src/app/registration/registration-page.spec.ts

@@ -1,6 +1,6 @@
 import { signal } from '@angular/core';
 import { fireEvent, render, screen } from '@testing-library/angular';
-import { of } from 'rxjs';
+import { of, throwError } from 'rxjs';
 import { AuthService } from 'auth';
 import { BffApiV1Service } from 'api-client';
 import { axe } from 'vitest-axe';
@@ -43,6 +43,19 @@ describe('RegistrationPage', () => {
     expect(await screen.findByText(/ontvangen/i)).toBeTruthy();
   });
 
+  it('shows an error and keeps the submit available when the BFF call fails', async () => {
+    const { post, providers: p } = providers(vi.fn().mockReturnValue(throwError(() => new Error('BFF rejected'))));
+    await render(RegistrationPage, { providers: p });
+
+    fireEvent.click(screen.getByRole('button', { name: /indienen/i }));
+
+    expect(post).toHaveBeenCalledTimes(1);
+    // The failure is surfaced (not swallowed), the confirmation is not shown, and the user can retry.
+    expect(await screen.findByRole('alert')).toBeTruthy();
+    expect(screen.queryByText(/ontvangen/i)).toBeNull();
+    expect(screen.getByRole('button', { name: /indienen/i })).toBeTruthy();
+  });
+
   it('has no WCAG 2.1 AA violations on the submit page', async () => {
     // The portal is Dutch; the real index.html sets lang. Set it here so the document-level
     // html-has-lang rule reflects the app, not the bare jsdom document.

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

@@ -21,13 +21,22 @@ export class RegistrationPage {
   protected readonly submitting = signal(false);
   protected readonly reference = signal<string | undefined>(undefined);
   protected readonly submitted = signal(false);
+  protected readonly failed = signal(false);
 
   submit(): void {
     this.submitting.set(true);
-    this.bff.postSelfServiceRegistrations().subscribe((accepted: SubmitAccepted) => {
-      this.reference.set(accepted.registrationId);
-      this.submitted.set(true);
-      this.submitting.set(false);
+    this.failed.set(false);
+    this.bff.postSelfServiceRegistrations().subscribe({
+      next: (accepted: SubmitAccepted) => {
+        this.reference.set(accepted.registrationId);
+        this.submitted.set(true);
+        this.submitting.set(false);
+      },
+      // Surface the failure instead of swallowing it: re-enable the button so the user can retry.
+      error: () => {
+        this.failed.set(true);
+        this.submitting.set(false);
+      },
     });
   }
 }