Make the now-playing bar's title/artist clickable and add a cover art lightbox

The song title links to its album, the artist name links to the artist
page, and clicking the cover art opens a larger version in a modal.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/database/library.go

@@ -492,6 +492,21 @@ func ListSongArtistNames(db *sql.DB, songID string) ([]string, error) {
 	return out, rows.Err()
 }
 
+// PrimarySongArtistID returns the ID of a song's first-credited artist
+// (position 0), or "" if the song has no linked artist — used to link
+// the now-playing bar's artist name to that artist's page.
+func PrimarySongArtistID(db *sql.DB, songID string) (string, error) {
+	var id string
+	err := db.QueryRow(`SELECT a.id FROM song_artists sa JOIN artists a ON a.id = sa.artist_id WHERE sa.song_id = ? ORDER BY sa.position LIMIT 1`, songID).Scan(&id)
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", nil
+	}
+	if err != nil {
+		return "", err
+	}
+	return id, nil
+}
+
 func nullInt(n sql.NullInt64) any {
 	if !n.Valid {
 		return nil