feat: artist sections, fav indicator, art scaling fix, volume popup fix

- Artist profile: collapsible Albums / EPs & Singles / Other sections
  keyed on release_type; fetches up to 200 albums per artist
- Favorites: starred icon on favorited tracks, context menu shows
  Add or Remove (not both); IDs cached when fav tracks are loaded
- Shuffle button: one-time shuffle via shuffleNow() without touching
  global shuffle flag, so double-click still plays in order
- Now-playing art: replaced setFixedHeight hack with ArtWidget that
  overrides hasHeightForWidth() — scales smoothly up and down, no min-size
- Volume popup: replaced QMenu (laggy, broken drag) with Qt::Popup QFrame;
  appears below button; fixed size locked at 100% label width

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

src/list/tracks.hpp

@@ -10,6 +10,7 @@
 #include <QVector>
 #include <QPair>
 #include <QString>
+#include <QSet>
 
 namespace List
 {
@@ -28,6 +29,11 @@ namespace List
         /// Called when the backend fires EV_TRACK_CHANGED so the playing row is highlighted.
         void setPlayingTrackId(qint64 id);
 
+        /// Populate favorite track IDs so the star indicator and context menu reflect fav status.
+        void setFavTrackIds(const QSet<qint64> &ids);
+        void addFavTrackId(qint64 id);
+        void removeFavTrackId(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);