Pits and stores are wells with scattered stones and count badges, the
viewer's store on the right. Below 600 px the board stands upright.
Playable pits preview their move on hover, animated pits are ringed and
the pit of the last move has a dashed ring.
Board components size themselves from a --board-unit CSS variable instead
of vw. By default it is 1vw, so the game page looks the same; with the new
width prop the same board renders small, e.g. in previews.
