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/playqueue.hpp

@@ -107,6 +107,14 @@ public:
         emit queueChanged();
     }
 
+    /// Shuffle the current queue once without changing the global shuffle flag.
+    void shuffleNow()
+    {
+        if (m_queue.isEmpty()) return;
+        shuffleQueue(m_index);
+        emit queueChanged();
+    }
+
     // ---- Play-next prepend queue (like "Add to queue" ----
 
     void addToQueue(const QJsonObject &track)