Add a warning pop up telling people that progress will be lost if they dont save the insight they're working on

Feature request

Is your feature request related to a problem?

Yes, I sometimes lose work because I'm working on an insight and I press the top left back arrow to go and do smth else (ex: create an action, cohort, etc) and I lose my work because I forget to save before leaving.

Describe the solution you'd like

A pop up warning people that they're about to lose any unsaved progress. I think that's the easiest way.

Describe alternatives you've considered

Automatic saving. But this seems like it might get confusing fast. As a user, I would prefer the first option.

Additional context

None

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

The issue does not name an entry point, component, file, or test. Start by locating the insight editor and the top-left back-navigation handler, then confirm the warning appears only when unsaved work would be lost and that existing navigation remains unchanged.