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/auth/src/lib/authenticated.guard.spec.ts

@@ -0,0 +1,42 @@
+import { signal } from '@angular/core';
+import { TestBed } from '@angular/core/testing';
+import { AuthService } from './auth.service';
+import { authenticatedGuard } from './authenticated.guard';
+
+class FakeAuth extends AuthService {
+  readonly isAuthenticated = signal(false);
+  readonly bsn = signal<string | undefined>(undefined);
+  login(): void {
+    /* spied in tests */
+  }
+  logout(): void {
+    /* noop */
+  }
+}
+
+function runGuard(authenticated: boolean) {
+  const auth = new FakeAuth();
+  auth.isAuthenticated.set(authenticated);
+  const loginSpy = vi.spyOn(auth, 'login');
+  TestBed.configureTestingModule({
+    providers: [{ provide: AuthService, useValue: auth }],
+  });
+  const result = TestBed.runInInjectionContext(() =>
+    authenticatedGuard(null as never, null as never),
+  );
+  return { result, loginSpy };
+}
+
+describe('authenticatedGuard', () => {
+  it('allows the route for an authenticated user', () => {
+    const { result, loginSpy } = runGuard(true);
+    expect(result).toBe(true);
+    expect(loginSpy).not.toHaveBeenCalled();
+  });
+
+  it('blocks and starts DigiD login when not authenticated', () => {
+    const { result, loginSpy } = runGuard(false);
+    expect(result).toBe(false);
+    expect(loginSpy).toHaveBeenCalledTimes(1);
+  });
+});