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/view/maincontent.cpp

@@ -112,3 +112,8 @@ void MainContent::updateArtistReleases(const QString &releaseType, const QJsonAr
 {
     m_artistView->setReleases(releaseType, items, hasMore, offset);
 }
+
+void MainContent::setFavArtistIds(const QSet<qint64> &ids)
+{
+    m_artistView->setFavArtistIds(ids);
+}