feat(context-menu): add "Add to Liked Songs" toggle to track context menus (task-276)

Add favorite toggle context menu item to library-browser, artists-browser,
and albums-browser views. The item checks favorite status asynchronously
when the menu opens and updates the label accordingly. On click, it
toggles the favorite via the existing favorites API, immediately updates
player.isFavorite for the currently playing track, and refreshes the
liked songs library view.

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

Author: pythoninthegrass