Blank window in the demo because of missing COOP header

[miquelvir]

When using https://react-oauth.vercel.app/ in Safari mobile, a blank screen appears and login does not work.

This is because of wrong COOP headers being used: https://developers.google.com/identity/gsi/web/guides/get-google-api-clientid#cross_origin_opener_policy

I had a similar issue in my web and fixed it by adding:

add_header Cross-Origin-Opener-Policy same-origin-allow-popups always;

(this is for nginx, if you use Next.js it will be different)

[miquelvir]

I added a hint about this to the docs as well: #382