Drag it. Drop it. If the save fails, it goes right back.

Every move here is optimistic — the card jumps instantly, then a simulated save runs in the background. Flip "simulate save failures" on below and drag a card: the UI reverts to the last saved state the moment the save rejects, no stale or half-moved cards left behind. Drag with a mouse, or tab to a card and use space + arrow keys.