argus (PR review): the thinking-flip useEffect left the button stuck
disabled if the /api/cancel POST itself failed while the turn was
still genuinely thinking, since nothing re-fires the effect. Switch
onCancelTurn's signature to () => Promise<void> and reset cancelBusy
in a .finally() on that promise instead — fires on success or
failure alike, matching app.js's original
postCancelTurn().finally(() => { btn.disabled = false; }).
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