feat(music): queue radio picks and save on request

The radio-browser lookup and the manual station creator no longer write .m3u files at pick time. Picks become url tracks in the queue, carrying the station name, uuid, and favicon as track properties, and play immediately. The display and cover-art layers read the properties first and fall back to the on-disk metadata, so existing station files keep working.

Saving is the normal playlist save, now on w. The radio feature's S=stop rebind had silently shadowed the old S=save binding. An all-stream queue saves into the MPD playlist dir, and the station name pre-fills the prompt with no -Radio suffix. A custom emitter writes the station metadata back out as .m3u comment lines, since the stock EMMS writer emits bare URLs and would lose names and cover art on reload.

I removed the write-at-pick machinery (station-m3u, write-stations, disambiguate-name, the -Radio filename suffix) and the orphaned safe-filename helper. An empty name at the save prompt now signals an error instead of writing a hidden .m3u.

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