fix: section alignment, pagination, fav state, playback error handling

**Section toggles left-aligned**
- Replace QToolButton with flat QPushButton for all section headers;
  QPushButton properly respects text-align: left in stylesheets

**Pagination via "Load More" button**
- QTreeWidget expands to fit all items so the scrollbar-based infinite
  scroll never triggered; replaced with an explicit "Load more…" button
  that appears when has_more is true and emits loadMoreRequested

**Favourite button reflects actual state**
- MainWindow preloads fav artist IDs on session restore (getFavArtists)
  and caches them in m_favArtistIds
- ArtistView receives the full set via setFavArtistIds() and checks
  it on every setArtist() call so the button starts gold if already faved
- Toggling updates the local cache immediately for back/forward nav

**Playback error → queue advances**
- player_loop now sets track_finished on Err (was only set on Ok(None)),
  so the toolbar's onTrackFinished handler advances to the next track
  instead of stalling on an unplayable track

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

src/mainwindow.hpp

@@ -53,6 +53,8 @@ private:
     QobuzBackend    *m_backend     = nullptr;
     PlayQueue       *m_queue       = nullptr;
     QVector<QPair<qint64, QString>> m_userPlaylists;
+    QSet<qint64> m_favArtistIds;
+    bool m_showFavArtistsOnLoad = false;
     MainToolBar     *m_toolBar     = nullptr;
     MainContent     *m_content     = nullptr;
     List::Library   *m_library     = nullptr;