feat(portal-openbaar): render the public register with search + empty state (refs #10)

Implement RegisterPage: load the full public register from the BFF on open, filter
by the search term via /openbaar/register?q=, and show a results table (referentie +
status), a loading indicator, and an empty-state message. Anonymous, same-origin
relative calls. Adds the app-shell spec. All openbaar lint/test/build green.

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

apps/openbaar/src/app/app.spec.ts

@@ -0,0 +1,14 @@
+import { provideRouter } from '@angular/router';
+import { render } from '@testing-library/angular';
+import { App } from './app';
+
+describe('App', () => {
+  it('renders the router outlet shell', async () => {
+    const { container } = await render(App, {
+      providers: [provideRouter([])],
+    });
+
+    // The shell is a thin host for routed pages (the RegisterPage owns the heading).
+    expect(container.querySelector('router-outlet')).toBeTruthy();
+  });
+});