Remove the system color-scheme fallback from the web UI theme boot path.
Fabro now uses a saved light/dark preference when present and otherwise
starts in dark mode by default. Add a regression test for the shared
theme selection helper and refresh the built web assets.
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.