[Bug] Auth: Google OAuth state cookie lost during onboarding personal step

[stilla]

Description

Users signing up via Google OAuth successfully authenticate but then encounter a Missing OAuth2 state cookie error when the /onboarding/personal form is submitted. From the user's perspective the error appears to happen when interacting with the date of birth field.

Steps to reproduce

1. Visit polar.sh and click Sign up
2. Choose "Sign in with Google" and complete Google OAuth
3. Land on /onboarding/personal and begin filling in the form (name, date of birth, etc.)
4. On form submission/navigation, get redirected to /auth?error=Missing+OAuth2+state+cookie
5. Retrying via "Sign in with Google" on the error page also fails; user is sent back to /onboarding/start

Expected behavior

User should proceed through onboarding without losing the OAuth state cookie.

Notes

• Related to [Detail Bug] Auth: Apple Sign In callback returns 500 instead of session-expired redirect when OAuth2 state is invalid/expired #12029 (fixed 500 → redirect for expired state), but the root cause — cookie loss during the flow — is still occurring for some users
• PR tf: add AUTHENTICATION_SESSION_COOKIE_DOMAIN and OAUTH2_SESSION_STATE_COOKIE_DOMAIN config #11956 introduced OAUTH2_SESSION_STATE_COOKIE_DOMAIN config; worth verifying this is correctly set for the onboarding redirect chain
• Likely a SameSite/cross-origin cookie handling issue affecting certain browsers or regions

_{Sent by @IsakPar021 from Account creation error logs.}