The cover was previously only wired through the UI hook; --play skipped it.
Also: Bun's fetch hangs in compiled binaries (Bun 1.3.8) — every cover
download silently failed in shipped builds, timing out against any host.
Cover staging now shells out to curl (present on macOS/Linux, 8s bound),
shared via src/utils/cover-art.ts so the UI and CLI paths stage the same
temp file and pass it to mpv as --cover-art-files (albumart track).
