Part 2 (Accounts). Apply after Part 1.

1. Extra package for LinkedIn sign-in (optional; Google works without it):
       composer require socialiteproviders/linkedin-openid
2. Google: create an OAuth client at console.cloud.google.com, add redirect URI
       https://YOURDOMAIN/auth/google/callback
   and set GOOGLE_CLIENT_ID / GOOGLE_CLIENT_SECRET in .env.
3. LinkedIn: create an app at linkedin.com/developers, enable "Sign In with LinkedIn using OpenID Connect",
   add redirect URI https://YOURDOMAIN/auth/linkedin/callback and set LINKEDIN_CLIENT_ID / LINKEDIN_CLIENT_SECRET.
   Sign-in buttons appear only when their keys are set.
4. Password reset emails use MAIL_* settings in .env. Test with a real address before launch.