test(portal-self-service): DigiD-guarded registration submit page (refs #67)

Scaffold libs/ui (NL Design System via Utrecht components) and libs/auth (DigiD OIDC
over angular-auth-oidc-client: mockable AuthService, provider, token interceptor,
authenticated guard). Failing component + axe tests for the RegistrationPage: it must
show the signed-in BSN, submit to the BFF (mocked api-client) and confirm, with no
WCAG 2.1 AA violations. The page is a stub, so the behaviour tests fail; green follows.

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

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

@@ -0,0 +1,9 @@
+import { Component } from '@angular/core';
+
+// STUB (red): no bsn, no submit — the component/axe tests fail until the green commit.
+@Component({
+  selector: 'app-registration-page',
+  imports: [],
+  template: '',
+})
+export class RegistrationPage {}