Protect Against Snippet Loss

Description

As I was assembling a longer snippet to use as an example, I accidentally navigated back in my browser, destroying the progress I had made. Yes, I should've used an external editor, but it'd be nice to have some protection.

Proposal

Let's implement support for window.onbeforeunload to avoid navigating away from the site or closing it by accident. Make sure this is implemented in the new Vue page that is behind the globally enabled feature flag as the old code will be removed soonish.