feat(music): playing-aware playlist landing and music-only adds

Opening the playlist used to jump to EMMS's selected track, which stays set while stopped, so the view opened deep in the list with point mid-row. It now lands on the playing track's row (beginning of line, upper third of the window) when a song is playing, and at the top of the list when stopped. Both entry points share the logic, and the playlist buffer gets hl-line so the current row stays findable on album art.

Also fixed: directory adds handed the raw tree to emms-add-directory-tree, which adds every file it finds. Cover art and liner notes became playlist rows. All three add paths now route through a filtered walk that keeps only accepted music extensions. The m3u loader applies the same filter to local paths (stream URLs pass through), so old playlists with saved cover lines stop re-importing them.

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