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/model/tracklistmodel.cpp

@@ -77,6 +77,36 @@ void TrackListModel::removeTrack(int row)
     endRemoveRows();
 }
 
+void TrackListModel::setFavIds(const QSet<qint64> &ids)
+{
+    m_favIds = ids;
+    if (!m_tracks.isEmpty())
+        emit dataChanged(index(0, ColTitle), index(rowCount() - 1, ColTitle),
+                         {Qt::DecorationRole});
+}
+
+void TrackListModel::addFavId(qint64 id)
+{
+    m_favIds.insert(id);
+    for (int r = 0; r < m_tracks.size(); ++r) {
+        if (m_tracks[r].id == id) {
+            const auto idx = index(r, ColTitle);
+            emit dataChanged(idx, idx, {Qt::DecorationRole});
+        }
+    }
+}
+
+void TrackListModel::removeFavId(qint64 id)
+{
+    m_favIds.remove(id);
+    for (int r = 0; r < m_tracks.size(); ++r) {
+        if (m_tracks[r].id == id) {
+            const auto idx = index(r, ColTitle);
+            emit dataChanged(idx, idx, {Qt::DecorationRole});
+        }
+    }
+}
+
 void TrackListModel::setPlayingId(qint64 id)
 {
     m_playingId = id;
@@ -128,6 +158,10 @@ QVariant TrackListModel::data(const QModelIndex &index, int role) const
         return QIcon(QStringLiteral(":/res/icons/media-track-show-active.svg"));
     }
 
+    if (role == Qt::DecorationRole && index.column() == ColTitle && m_favIds.contains(t.id)) {
+        return QIcon(QStringLiteral(":/res/icons/starred-symbolic.svg"));
+    }
+
     if (role == TrackIdRole)         return t.id;
     if (role == TrackJsonRole)       return t.raw;
     if (role == HiResRole)           return t.hiRes;