chore(android): drop redundant !! on PlaylistRef.systemVariant

Line 264 already null-checks playlist.systemVariant in the if
condition. PlaylistRef is a data class with a val backing field,
so the smart cast narrows it to String inside the branch — the
!! on line 265 was a no-op the Kotlin compiler was warning about.

android/app/src/main/java/com/fabledsword/minstrel/home/ui/HomeScreen.kt

@@ -262,7 +262,7 @@ class HomeViewModel @Inject constructor(
             val detail = try {
                 withTimeout(PLAYLIST_FETCH_TIMEOUT_MS) {
                     if (playlist.refreshable && playlist.systemVariant != null) {
-                        playlistsRepository.systemShuffle(playlist.systemVariant!!)
+                        playlistsRepository.systemShuffle(playlist.systemVariant)
                     } else {
                         playlistsRepository.refreshDetail(playlist.id)
                     }