A user presses Space on “Email alerts.” The switch immediately turns on. The request fails, the UI silently flips back, and a screen-reader user receives no explanation. Optimistic UI made the happy path faster by making the failure path ambiguous. The control needs four truths, not one Boolean: type ToggleState = { confirmed : boolean ; desired : boolean ; requestId : number | null ; error : string | null ; }; confirmed is...