Adds the features shipped since the list was written (Identify Track,
Import from URL, andTunes, art sources, New Tracks, merge improvements,
Cassette) without touching the existing lines, and a step-by-step guide
at the end: installing Syncthing, inviting a friend, choosing what to
share, cassetting songs, following playlists and troubleshooting. Icons
and screenshots in docs/cassette/ are rendered from the app's own
painting code against the synthetic dev library.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
