address review: cancellation-safe star, one star handler, focus logic tested

- setStar no longer treats cancellation as a rejection: rethrow CancellationException
  instead of toasting/rolling back, and roll back under NonCancellable on a real failure
- serialise setStar so overlapping toggles cannot clobber each other's rollback
- the menu star item reverts its label when the request fails (setStar now returns the
  result), so the open menu cannot drift from the row
- ContextMenuHost.dismiss clears the actions so a popped screen's ViewModel is released
- one shared setStar from the app shell instead of five per-ViewModel copies
- extract the nav-focus decision into focusPops and unit-test it

app/src/main/kotlin/com/subtracks/ui/library/AlbumDetailViewModel.kt

@@ -8,7 +8,6 @@ import com.subtracks.data.model.Disc
 import com.subtracks.data.model.Song
 import com.subtracks.data.repo.LibraryRepository
 import com.subtracks.data.repo.SourceRepository
-import com.subtracks.data.source.StarType
 import com.subtracks.playback.PlaybackController
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.Flow
@@ -54,12 +53,4 @@ class AlbumDetailViewModel(
             playbackController.shuffleAlbum(sourceId.first(), albumId)
         }
     }
-
-    fun setStar(
-        type: StarType,
-        id: String,
-        starred: Boolean,
-    ) {
-        viewModelScope.launch { libraryRepository.setStar(type, id, starred) }
-    }
 }