What does this PR do?

Is this a bugfix or an improvement?;

Enhancement.

Explain the changes this PR brings to the project;

Album art is added to the notifications, if notifications are enabled.

Mention a issue (adding the link, of course) if there is any relationship;

35 is partially fixed by this PR.

Insert some print screen of the result, if it changes the UI.

Explain how to test in localhost and/or with the build app (what to do, which songs if necessary, etc).

Run the app using yarn start and turn on notifications in System Preferences -> Spotify.

Is there any part of the code that deserves more attention?;

Not that I can see as of yet.

Any technical debt produced by this code? Something could be done better but it wasn't? Why?.

Don't think so, very little code was changed.

Thanks!