Deduplicates the hand-rolled coalescing/caching maps, the Plex client cast,
the missing-serverId event guard and the progress-failure backoff, and drops
the MusicPlaybackService availability gate, which could never fail in
production.
Introduces shared seams for paginated views, D-pad reorder, media control
routing, async singletons and the device method channel, then points the
open-coded copies at them.
Also removes unused models and duplicated provider/server plumbing, folds
the twice-implemented artifact store in the server, and factors the
repeated Flutter toolchain prologue in CI into a composite action.
edde746