Dual-source queue sync

A player engine keeps its own timeline of media items — ids, uris, metadata — while the app keeps a richer domain list for the UI: artwork, artists, like state, everything the engine has no field for. Both are the queue. Only one can be the authority on order, and it has to be the engine, because the engine is what actually plays next.

So the domain list is re-derived from the timeline after any engine-side change: