Implement the web-first install experience across the server, CLI, API spec,
web app, and packaged SPA assets.
This also removes test-side process env mutation by pushing env-dependent
decision points behind explicit helpers and test wiring.
Replace the unsupported Bun.watch call in the SPA build script with
node:fs.watch so `bun run dev` keeps running in local development.
Add a regression test that verifies watch mode stays alive until
interrupted.
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.
