Tapping Instant Mix on Plex often did nothing: the /playQueues POST used no
endpoint failover, createPlayQueue swallowed every error into null,
fetchInstantMix mapped that to an empty list, playInstantMix returned
silently on empty tracks, and the errors stream's only listener is the
now-playing screen, which is not mounted at tap time (#2141).
The play-queue POST now rides the shared endpoint failover (replaying it is
safe: an orphaned duplicate queue on the server is inert), createPlayQueue
and fetchInstantMix propagate typed errors, playInstantMix reports
started/empty/superseded and throws fetch failures, and the tap site shows
a translated snackbar for a failed or empty mix. A failed collection or
playlist queue launch now also reaches the real failure snackbar instead of
the misleading "no items" path.
edde746
a1d8ab5210