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>

libs/ui/src/lib/ui.spec.ts

@@ -0,0 +1,7 @@
+import { UtrechtComponentsModule } from '../index';
+
+describe('ui barrel', () => {
+  it('re-exports the NL Design System (Utrecht) module', () => {
+    expect(UtrechtComponentsModule).toBeTruthy();
+  });
+});