Add crossfade support

[juanqp07]

It would be great to have crossfade support when playing music.

Instead of stopping one track and immediately starting the next, Chora could gradually fade out the current track while fading in the next one, creating a seamless transition between songs.

Suggested behavior

• Add a Crossfade option in the playback/settings screen.
• Allow the user to enable or disable it.
• Allow configuring the crossfade duration (for example, 2–12 seconds).
• Apply it automatically when moving between tracks in the queue.
• Ideally, make it work on both Android mobile and Android TV.

This would be especially useful for continuous listening and genres where seamless transitions are desirable.

Thanks for the great work on Chora!