fix(desktop): close the backend update overlay on success; error on no-return

Three rough edges in the remote backend apply flow:
- On success the overlay dropped to IDLE, briefly re-rendering the pre-install
  'update available' view and then the generic 'you're all set' before settling.
  Close the overlay outright once the backend is confirmed back instead of
  bouncing through the idle view.
- If the backend never came back (a failed restart), the flow still reported
  success. waitForBackendReturn now returns whether the backend answered;
  finishBackendApply surfaces an error when it didn't.
- The up-to-date copy said 'you're running the latest version', conflating
  client and backend. Backend target now reads 'the backend is running the
  latest version' — the client's own version is a separate pill.

apps/desktop/src/app/updates-overlay.tsx

@@ -189,7 +189,7 @@ function IdleView({
   if (behind === 0) {
     return (
       <CenteredStatus
-        body={u.latestBody}
+        body={target === 'backend' ? u.latestBodyBackend : u.latestBody}
         icon={<CheckCircle2 className="size-7 text-emerald-600 dark:text-emerald-400" />}
         title={u.allSetTitle}
       />