Fix game result banner not showing for the winning player
The winning player's result banner (You win!) was not rendering after
delivering checkmate. The game.result getter relied on gameResult(chess)
which reads from a chess.js class instance mutated in place -- Svelte 5
runes don't track mutations on class instances, so the template never
re-evaluated. Fix by explicitly calling game.setResult() in writeMove()
which sets the storedResult $state variable, triggering reactivity.