New CountdownOverlay — a full-screen, click-through, all-spaces screen-saver panel that punches each numeral in with a scale-pop, synced to the existing voice count-in in beginCountIn (3/2/1 at 0.7s, 2.1s total). Cleared on the downbeat and on cancel. Kept separate from FocusFlashOverlay because that overlay fades its whole window for the red charge glow, so the numbers need their own full-brightness layer on top.