Adds some simple monitoring UI for end-users + alert when >80% memory/CPU. NOTE: Currently this is only behind the isDev flag, so it will not be in the built app; only in dev mode.

This should make it easier to identify why Reflex starts to feel laggy, so you can see that when you have 5 screen sizes displayed all at full-height, you'll be able to see how your system's usage also increases into the yellow/red. Sometimes you may want to keep using it, even if laggy, so that you can see some specific thing across many screen sizes, so we won't prevent you from doing it, but we will offer a lifeboat if you want to turn back.