Dev-only quick sign-in for testing preview and local deployments

New

• Added a /dev-login page (PropChain, Marketing, and this documentation site) that lets you sign in with a plain email and password directly on that app, without going through the shared login app. It only works on preview and local environments, never in production, and is meant for developers checking signed-in pages on a preview link where the normal cross-app sign-in doesn't work.

Improvements

• Removed the unused preview-domain names from the sign-in redirect safety list, now that the dev-only quick sign-in above covers testing signed-in pages on previews.
• PropChain and Marketing's "Sign in" link now automatically sends you to the right place: the shared login app in production, or the new dev-only quick sign-in on preview and local environments.

Fixes

• Visiting a signed-in-only page in PropChain while signed out now sends you to the right place on preview and local environments too, instead of always trying the shared login app (which can't send you back on anything but the live site).
• Signing in with the dev-only quick sign-in on PropChain now takes you to the dashboard instead of a page that didn't exist.