feat(music): always dock the playlist at the bottom

The F10 playlist docked as a right-side column when the frame was wide enough, splitting the frame three ways. That's annoying and often unexpected, so the frame's shape no longer decides where the playlist goes.

I deleted the chooser (cj/--music-playlist-side) and the right-dock width knobs rather than leaving them wired to nothing, and a test asserts they're gone. cj/preferred-dock-direction stays, since ai-term and eat still dock by frame shape.

The old side-chooser test is replaced by one that stubs cj/side-window-display and asserts what the toggle asks for, so it runs headless.

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