Wire the sidepanel backend ahead of its UI: select-artist loads the chosen
artist via artist::load_artist (reusing the /artist/page release_type
classifier per the user's decision) and apply_selected_artist maps its
release sections into selected-artist-sections (DiscoverSections) + total;
a FavoriteArtistAlbum artwork target fills the section covers. The
artists-set-view handler flips grid<->sidepanel (persisted as artists_view)
and clears the selection on the way back to grid. card_to_item is now
pub(crate) for reuse.
cc
vicrodh
832f257e5a