fix(web): drop optimistic updates and disable inert Start-minimized toggle

- AutoLaunchSettings: remove pre-await state writes; backend response is
  now the sole writer to openAtLogin/startMinimized state, eliminating
  the flicker caused when optimistic values diverged from OS truth
- Add busy flag across IPC round-trip; both Toggles receive disabled={busy}
- Start-minimized Toggle additionally receives disabled={!openAtLogin},
  preventing interaction when it would have no effect
- Toggle.tsx: add optional disabled prop (forwarded to <button disabled>)
  with cursor-not-allowed opacity-50 visual feedback; non-breaking for
  all existing callers

packages/web/src/components/ui/Toggle.tsx

@@ -1,16 +1,18 @@
 interface ToggleProps {
   enabled: boolean;
   onChange: (enabled: boolean) => void;
+  disabled?: boolean;
 }
 
-export function Toggle({ enabled, onChange }: ToggleProps) {
+export function Toggle({ enabled, onChange, disabled }: ToggleProps) {
   return (
     <button
       type="button"
       onClick={() => onChange(!enabled)}
+      disabled={disabled}
       className={`relative inline-flex h-6 w-11 items-center rounded-full transition-colors flex-shrink-0 ${
         enabled ? 'bg-accent-primary' : 'bg-interactive-muted'
-      }`}
+      }${disabled ? ' cursor-not-allowed opacity-50' : ''}`}
     >
       <span
         className={`inline-block h-4 w-4 rounded-full bg-white transition-transform ${