Add fixed playlist playback controls

[keilogic]

Summary

• add an iPhone/iPad playlist-detail fixed playback panel that appears while a player queue is active
• provide large previous/play-next controls without shuffle/repeat controls
• add an inline 1-100 volume slider with down/up buttons and numeric level display
• keep the playlist table clear of the panel by extending the bottom safe-area inset above the existing mini-player
• leave Mac Catalyst unchanged because the Mac design uses a different playback-control surface

Refs #312, items 2 and 3.

Validation

• git diff --check HEAD~1..HEAD

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-control scope from #312, PayPal route: https://www.paypal.com/paypalme/aogkft