Move the browser sign-in page off the backend /auth namespace so direct
navigation works, update Rust and frontend redirects to /login, and add
tests that lock the split between SPA login UI and backend OAuth endpoints.
Replace the old React Router SSR setup with a static SPA build served by
fabro-server, move setup and GitHub auth handling into Rust, and update the
default local web URL and stale Arc-era references to match the Fabro name.
