give `corpus-view.ts` an interlock to warn about gitcasso running

- when testing gitcasso using `pnpm corpus`
- if you have the gitcasso extension installed in your browser (dev or prod, doesn't matter)
- then you will get extremely misleading results
- right now we warn in the docs
- https://github.com/diffplug/gitcasso/blob/45ea112f09ad67947f0c2d10394251356e5599b4/CONTRIBUTING.md#L89-L91
- it would be much better if we could somehow detect that the browser extension was running, and give a big fat warning if that is the case
- maybe if `content.ts` runs on `localhost` we add some header into the DOM
- and then `corpus-view.ts` has some js that looks for that and makes a big warning div
- then we can remove the warning in `CONTRIBUTING.md`
- I have already lost hours to this mistake, new contributors are guaranteed to lose more

Start by reading corpus-view.ts and content.ts, then review the warning section in CONTRIBUTING.md. Run pnpm corpus with the gitcasso browser extension enabled to reproduce the misleading results. Done means the corpus view shows a prominent warning when the extension is active on localhost, and the redundant documentation warning can be removed.

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