feat(portal-self-service): implement the DigiD registration submit page (refs #67)

RegistrationPage shows the signed-in BSN and submits to the BFF via the generated
api-client, confirming with the returned reference; built from NL Design System
(Utrecht) components. Wire the guarded route + app providers (DigiD OIDC + token
interceptor + HttpClient), the NL DS theme, and lang=nl. Component tests
(Testing Library) + axe (WCAG 2.1 AA) pass; a guard test covers libs/auth. Replace
the demo eslint depConstraints (scope:shop/shared) with a permissive default.

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

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

@@ -0,0 +1,22 @@
+<main utrecht-document class="utrecht-theme">
+  <utrecht-article>
+    <utrecht-heading-1>Zelfservice — BIG-registratie</utrecht-heading-1>
+
+    @if (submitted()) {
+      <p utrecht-paragraph role="status">
+        Uw registratie is ontvangen. Referentie: {{ reference() }}.
+      </p>
+    } @else {
+      <p utrecht-paragraph>U bent ingelogd met BSN {{ bsn() }}.</p>
+      <button
+        utrecht-button
+        appearance="primary-action-button"
+        type="button"
+        [disabled]="submitting()"
+        (click)="submit()"
+      >
+        Registratie indienen
+      </button>
+    }
+  </utrecht-article>
+</main>