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/App/AppGraph.swift

@@ -20,6 +20,7 @@ final class AppGraph {
     let cache: MediaCache
     let eq: EqController
     let playback: PlaybackManager
+    let stemPlaylists: StemPlaylists
     let downloads: Downloads
     let coverage: StemCoverage
     let nowPlaying: NowPlaying
@@ -59,6 +60,7 @@ final class AppGraph {
         cache = MediaCache(dir: media, session: mediaSession) { Int64(settings.cacheLimitGb) << 30 }
 
         eq = EqController(dir: support)
+        stemPlaylists = StemPlaylists(dir: support)
         playback = PlaybackManager(settings: settings, plex: plex, stems: stems, cache: cache, eq: eq, dir: support)
         downloads = Downloads(settings: settings, plex: plex, stems: stems, cache: cache, net: net, dir: support)
         coverage = StemCoverage(plex: plex, stems: stems, settings: settings, dir: support)