feat(kiosk): Now Playing centerpiece + NPB miniplayer/immersive consistency

KioskNowPlaying: a 50/50 full screen (big cover + title/artist + AudioStamp
+ SeekBar + the reused 7-control TransportControls on the left, the Up-Next
queue on the right), reading NowPlayingState/QueueState and calling
QueueState.panel-opened() on init. New ContentView.now-playing route; the
NavRail Now Playing tile navigates there instead of opening immersive. Also
hide the Miniplayer + Immersive items from the NPB view menu while in the
kiosk profile (todos o ninguno) so it is not a second-window/overlay mix.
Validated via slintcheck.

cc

crates/qbz-ui/ui/state.slint

@@ -979,7 +979,7 @@ export global DragActions {
 }
 
 // Which content view the shell shows.
-export enum ContentView { home, album, artist, artist-releases, settings, search, musician, label, label-releases, award, award-albums, location, favorites, mix, playlist, discover-browse, recent-albums, playlist-manager, offline-manager, blacklist-manager, local-library, local-album, mixtapes, collections, mixtape-detail, discography-builder, purchases, purchase-album }
+export enum ContentView { home, album, artist, artist-releases, settings, search, musician, label, label-releases, award, award-albums, location, favorites, mix, playlist, discover-browse, recent-albums, playlist-manager, offline-manager, blacklist-manager, local-library, local-album, mixtapes, collections, mixtape-detail, discography-builder, purchases, purchase-album, now-playing }
 
 // In-app toast (transient notification). Mirrors Tauri's toastStore:
 // a single toast at a time (a new one replaces the current), auto-hidden