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/Nav.swift

@@ -18,7 +18,7 @@ enum Route: Hashable {
     case Artists, Albums, Songs, Playlists
     case Eq, EqFilters(profileId: String), EqFilterPage(profileId: String, filterId: String)
     case PlexAccount, StemServer, Quality, Storage, Appearance
-    case ArtistPage(Artist), AlbumPage(Album), PlaylistPage(Playlist)
+    case ArtistPage(Artist), AlbumPage(Album), PlaylistPage(Playlist), StemPlaylistPage(String)
 }
 
 /// Tabs, their stacks and the Now Playing sheet: the things any screen can ask of the shell.