fix: lazy-load genre/playlist views and cap playlist search scrolling

- Defer eager-load scrollbar checks to next event loop iteration via
  QTimer::singleShot(0), fixing initial load not filling the viewport
- Playlist search: eagerly fill viewport, then show "Load more playlists…"
  button instead of infinite scroll to avoid loading thousands of results
- Increase search page size from 8 to 25
- Featured/discover playlists keep auto-scroll behavior unchanged

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

src/view/genrebrowser.hpp

@@ -70,6 +70,8 @@ private:
     QStackedWidget *m_resultsStack = nullptr;
     AlbumListView *m_albumList = nullptr;
     QTreeWidget *m_playlistList = nullptr;
+    QPushButton *m_loadMorePlaylistsBtn = nullptr;
+    bool m_searchViewportFilled = false;
     BrowseMode m_mode = BrowseMode::Genres;
     bool m_genresLoaded = false;
     int m_lastGenreComboIndex = 0;