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was thinking that this could be a cool feature - make a copy of the selected playlist in my music, then allow me to add/remove tracks from it
You can do that in the desktop client by selecting all tracks in a playlist using ctrl-a/cmd-a and dragging them onto "Songs". This won't work for local files though.
Ah, in that case you can just save the tracks one at a time from the context menu. The idea's been suggested here so be sure to vote for it 🙂