feat(slint): minimal playback wiring + Queue, plus detail/Settings fixes

Playback (minimal, album-focused):
- playback.rs controller: play an album (set queue + start audio),
  play a single track, enqueue, play a queue index, transport
  (play/pause, next, previous, seek, volume)
- the audible step uses Player::play_track (fetch -> download -> play);
  the protected audio backend is untouched
- a 450ms poll loop pushes position/progress/times/playing onto
  NowPlayingState and auto-advances at end of track
- NowPlayingState / QueueState globals; NowPlayingBar bound to real
  state (song card, seek bar, transport, volume); QueueSidebar lists
  the queue and rows play on click
- a played track is recorded into the recently-played store

Detail views:
- album description and artist biography stay with the metadata,
  filling the width to the right of the artwork (not below the header)
- header nav tabs gained a dropdown caret and 2px side padding

Settings > Playback defaults are now opt-out: show context icon,
restore session, resume position and stream-uncached default on;
streaming-only stays opt-in; initial buffer defaults to 2s.

Known gaps: queue/now-playing artwork not yet threaded (Image is not
Send); autoplay-mode preference not consulted by auto-advance; buffer
progress line stays at 0.

cc

crates/qbz-slint/ui/app.slint

@@ -10,7 +10,7 @@ import { LoginScreen } from "login/LoginScreen.slint";
 import { AppShell } from "shell/AppShell.slint";
 
 // Re-export the state globals so the Rust layer can populate them.
-export { HomeState, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions } from "state.slint";
+export { HomeState, AlbumState, ArtistState, NavState, SessionState, SettingsState, AlbumActions, NowPlayingState, QueueState } from "state.slint";
 
 // Which top-level screen is shown. The app starts on `splash` while it
 // restores a saved session, then resolves to `shell` or `login`.