feat: playlist art fix + Play/Shuffle buttons in context header

- Fix playlist art: API returns images/images150/images300 arrays, not
  image_rectangle; update Rust model and Qt header to use images300 →
  images150 → images with fallback
- Add Play (▶) and Shuffle (⇄) buttons to the album/playlist header;
  Play starts from track 1, Shuffle enables shuffle mode and plays from
  a randomised position

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/list/tracks.hpp

@@ -28,6 +28,10 @@ namespace List
         /// Called when the backend fires EV_TRACK_CHANGED so the playing row is highlighted.
         void setPlayingTrackId(qint64 id);
 
+        /// Start playing all tracks in the current view from the beginning.
+        /// If shuffle is true, enables shuffle mode before starting.
+        void playAll(bool shuffle = false);
+
         /// Set which playlist is currently displayed (0 = none).
         /// isOwned controls whether "Remove from this playlist" is shown.
         void setPlaylistContext(qint64 playlistId, bool isOwned = false);