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).

lyrics_test.go

@@ -21,6 +21,32 @@ func TestCleanLyrics(t *testing.T) {
 	}
 }
 
+// Plain text must parse to nil: that is the signal to scroll continuously rather than highlight
+// lines on timings nobody measured.
+func TestParseLRC(t *testing.T) {
+	if got := parseLRC("Ihan tavallista tekstiä\ntoinen rivi"); got != nil {
+		t.Fatalf("plain text parsed as synced: %v", got)
+	}
+
+	lines := parseLRC("[00:11.74] Ensimmäinen\n[01:02] Toinen\n[00:05.5] Aikaisempi\nrivi ilman aikaa")
+	if len(lines) != 3 {
+		t.Fatalf("got %d lines, want 3 — untimed lines are dropped", len(lines))
+	}
+	// Sorted by time, whatever order the file had.
+	if lines[0].At != 5.5 || lines[0].Text != "Aikaisempi" {
+		t.Fatalf("first line is %+v, want 5.5s Aikaisempi", lines[0])
+	}
+	if lines[1].At != 11.74 || lines[2].At != 62 {
+		t.Fatalf("timestamps parsed as %v and %v, want 11.74 and 62", lines[1].At, lines[2].At)
+	}
+
+	// A refrain can carry several timestamps on one line, and each is its own occurrence.
+	rep := parseLRC("[00:10.00][01:10.00] Kertosäe")
+	if len(rep) != 2 || rep[0].At != 10 || rep[1].At != 70 {
+		t.Fatalf("repeated stamps gave %+v, want two occurrences", rep)
+	}
+}
+
 // The lookup is a suggestion, so "nothing found" is a normal answer rather than an error, and a
 // synced hit always beats a plain one.
 func TestFetchLyrics(t *testing.T) {