Previous commit took the user's (O-) / (-O) shorthand literally and put
those glyphs as the button label. They were describing the visual states
of a real toggle switch, not text to display.
Now: pill-shaped track with a sliding circular thumb, "default" and
"overwrite" labels flanking it (the active side brightens). The button
carries role="switch" + aria-checked for accessibility. Click anywhere
on the track to flip and mark the score dirty.
