fix(ui): warn on stderr when --ui requested without embedded UI (#350)

A binary built without the embedded frontend (CBM_EMBEDDED_FILE_COUNT == 0)
accepted --ui=true, persisted it to config, but never started the HTTP server
— the only signal was a ui.no_assets warning routed to the log-file sink, which
a user running `--ui=true` on a terminal never sees. Detect an explicit
--ui=true on a non-UI binary and print a clear stderr message pointing to the
UI release asset / cbm-with-ui build, so the silent no-op is explained.