Adds _radioSeedId state and a module-level $effect.root that triggers a
/api/radio?seed_track=…&exclude=… fetch when queue consumption reaches
80%, appending new tracks without resetting the radio seed. Clears seed
on any non-radio enqueue (playQueue/enqueueTrack/enqueueTracks). Five
new vitest cases cover the trigger threshold, append logic, in-flight
guard, and manual-enqueue reset path.
bvandeusen
d8b955ff0c