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/Plex/PlexModels.swift

@@ -405,7 +405,7 @@ extension PlexItem {
     func toArtist() -> Artist { Artist(ratingKey: ratingKey, title: title, sortTitle: titleSort ?? title, thumb: thumb) }
 
     func toPlaylist() -> Playlist {
-        Playlist(ratingKey: ratingKey, title: title, thumb: composite ?? thumb, trackCount: leafCount ?? 0,
+        Playlist(ratingKey: ratingKey, title: title, thumb: thumb ?? composite, trackCount: leafCount ?? 0,
                  durationMs: duration ?? 0, smart: smart ?? false)
     }
 }