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>

libs/ui/src/index.ts

@@ -0,0 +1,10 @@
+// NL Design System building blocks — Utrecht is NL DS's reference Angular implementation
+// (docs/frontend-decisions.md). The portals depend on the design system through this one lib.
+// The design tokens/theme CSS is imported once, at the app level (apps/*/src/styles.css).
+export {
+  UtrechtArticle,
+  UtrechtButtonAttr,
+  UtrechtDocument,
+  UtrechtHeading,
+  UtrechtParagraph,
+} from '@utrecht/component-library-angular';

libs/ui/src/test-setup.ts

@@ -0,0 +1,5 @@
+import '@angular/compiler';
+import '@analogjs/vitest-angular/setup-snapshots';
+import { setupTestBed } from '@analogjs/vitest-angular/setup-testbed';
+
+setupTestBed({ zoneless: false });