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>
