Add settings playlist track sync

[keilogic]

Summary

• add a Library settings button to retrieve track lists for every current playlist
• reuse the existing all-playlists sync flow: refresh playlist metadata, then fetch each playlist's tracks from the server
• add a confirmation alert and running-state guard so the action is not started twice

Refs #312, item 5.

Validation

• git diff --check

I could not run Xcode/Swift tests from this Windows environment because Xcode and Swift tooling are not installed here.

If this is accepted for the paid playlist retrieval scope from #312, PayPal route: https://www.paypal.com/paypalme/aogkft