feat(now-playing): add immersive full-screen karaoke overlay

[TavoNiievez]

A full-window "Now Playing" screen: full-size album cover on the left, centered full-size synced karaoke lyrics on the right, over a blurred copy of the cover. Opened by an expand button on the player cover (both the right card and the bottom bar), closed with its chevron-down button or Esc.

Reuses the existing player stack rather than adding new logic:

• the playback + scrub stores drive transport and the progress slider;
• ProgressSlider / VolumeControl / useITunesCover are imported from the player bar; the transport cluster is inlined (the two bars are left untouched so their differing layouts don't regress);
• the LyricsBody / useLyricsView karaoke engine already highlights the active line, auto-scrolls, and seeks on line click.

Details:

• New non-persisted store (now-playing.ts) holds the open flag, so it never survives a reload or leaks across windows via the shared layout key.
• Overlay is portaled to and the app root is marked inert while open (background out of the tab order + a11y tree) - modal focus with focus-in/restore and no focus-trap library. Reused components are forced into a dark token context so they stay legible over the dark backdrop; a ~55% scrim + text-shadow keep lyrics readable over bright covers; reduced-motion is respected on the open animation.
• Positioned below the 36px title bar so the OS window controls stay usable; auto-closes (and resets the flag) when the queue empties; the expand button is suppressed in the floating window.