Clicking an artist (from the album page's artist line) opens the
artist detail view: a circular portrait, name, biography, action
buttons, a Popular Tracks list, and a Discography carousel.
- artist.rs: fetches the artist page via QbzCore::get_artist_page and
maps name, biography (HTML-stripped), portrait URL, top tracks and
releases; the portrait loads through the shared image cache.
- ArtistPageView.slint reuses AlbumCard for the discography (through
the Carousel) and AlbumTrackItem for the track list.
- NavState becomes a three-way ContentView (home / album / artist) and
AppShell routes the content region accordingly. The album page's
artist line and the artist page's release cards are navigable.
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