What would you want/need a desktop app for? If you use Chrome (and yes, I'm aware some people use Firefox) you can install it as an App that way, so it appears in your start menu/finder. It can cast to your local devices etc.

I can't think of a single reason I'd want/need a standalone app over having the Chrome version of the app, which to all intents and purposes appears as a standalone app anyway.

So I'm curious, what's the use-case for a Desktop App to stream music? Even with the webapp you can download music for offline play.