[feat] Allow canceling "Start Flow..." window

Once you choose Start Flow…, there’s no obvious way to cancel out of the window.

There’s no close button, cancel button, or Esc handling, and clicking elsewhere on the screen takes a couple seconds to trigger a close (and bringing the mouse back in before that time is up, which you’d do if you’re still trying to get it to close, keeps it open).

What about letting the escape key cancel out of that window?