Adds androidx.palette dependency and a rememberDominantColor()
helper that pulls the cover bitmap via Coil's singleton loader
(shares the cache with the on-screen AsyncImage), runs Palette
extraction on Dispatchers.Default, and animates the resulting
color with animateColorAsState so track changes tween smoothly.
NowPlayingScreen wraps the body in a Box with a vertical gradient
(0% dominant @ 55%α → 45% dominant @ 18%α → 100% scheme.background)
and lets the Scaffold's containerColor go transparent so the
gradient shows through. Falls back to a near-transparent gradient
on bitmap-load failure so the screen never sits on flat black.
Hero transition (MiniPlayer → NowPlaying cover) and cover
preload still pending — those land in a follow-up commit.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>