iOS 1.0.8: stem playlists, karaoke softness slider in lyrics

- Playlists tab splits into Plex and Stem. Plex playlists are unchanged and
  now show their custom poster when one is set.
- Stem playlists live on the device; each song keeps its own mix. Add from
  any song or collection menu, or copy a whole Plex playlist.
- Queue items can carry a mix, which wins over the song's saved mix.
- Softness slider under the Vocals picker in lyrics when set to Soft.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/Stemify/UI/RootView.swift

@@ -65,6 +65,7 @@ struct RootView: View {
         case .ArtistPage(let a): ArtistScreen(artist: a)
         case .AlbumPage(let a): AlbumScreen(album: a)
         case .PlaylistPage(let pl): PlaylistScreen(playlist: pl)
+        case .StemPlaylistPage(let id): StemPlaylistScreen(id: id)
         }
     }
 }