studio/frontend: keep shutdown dialog open when server-stop fails

`AlertDialogAction` from Radix auto-closes the AlertDialog on click
(documented behavior). The shutdown handler then runs async, and on
error calls `toastError(...)` + `setStopping(false)` to reset the
button label for a retry, but the dialog has already vanished by
then. The user sees a flash + a toast and has to re-navigate through
the user-menu to try again.

Fix: call `event.preventDefault()` in the AlertDialogAction onClick
so the dialog stays open while the request is in flight. On success
the handler replaces document.body wholesale, so dialog open-ness
doesn't matter. On failure the dialog stays open with the reset
"Stop server" label, ready for retry.

Probe: scripts/r6_shutdown_dialog_probe.py — intercepts
/api/shutdown to return 500, asserts dialog still open + button
label resets to "Stop server".

studio/frontend/src/components/shutdown-dialog.tsx

@@ -72,7 +72,15 @@ export function ShutdownDialog({
         <AlertDialogFooter>
           <AlertDialogCancel>Cancel</AlertDialogCancel>
           <AlertDialogAction
-            onClick={handleStop}
+            onClick={(event) => {
+              // AlertDialogAction auto-closes the dialog by default.
+              // On the shutdown error path we toast + reset `stopping`
+              // to let the user retry, which only works if the dialog
+              // stays open. preventDefault keeps it open; on success
+              // the handler replaces document.body anyway.
+              event.preventDefault();
+              void handleStop();
+            }}
             disabled={stopping}
             variant="destructive"
           >