Follow the song in the lyrics

Synced LRC highlights the playing line, keeps it centred and seeks on click.
Plain text scrolls continuously with a nudge knob instead — a highlight on
guessed timings turns guaranteed drift into what looks like a bug. Seuraa
kappaletta turns following off without losing the highlight, and scrolling by
hand turns it off too.

Fixes the scroll landing in the wrong place (offsetTop measured from a
different coordinate space than the box it was applied to) and the fader
shifting the deck sideways at score 100 (auto-sized grid columns plus a
readout spanning both).

songs.go

@@ -160,6 +160,10 @@ type songDetail struct {
 
 func (s *songDetail) Locked() bool { return s.ReviewCount > 0 }
 
+// Synced lyrics get a line-by-line highlight; plain text scrolls continuously instead, because a
+// highlight on guessed timings makes every second of drift look like a bug.
+func (s *songDetail) LyricLines() []lyricLine { return parseLRC(s.Lyrics) }
+
 func (a *app) song(ctx context.Context, viewerID, songID int64) (*songDetail, error) {
 	var d songDetail
 	err := a.pool.QueryRow(ctx, `select`+songColumns+`,