Make fabro install the only supported GitHub App setup path. This removes
HTTP endpoints and browser routes that mutated local server config, rewrites
/setup as an operator instructions page, and aligns the installer manifest
with the live GitHub OAuth callback and setup URLs.
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.
