feat(provider): add ThunderID provider

[brionmario]

☕️ Reasoning

ThunderID is an open-source, self-hosted identity and access management platform.

This PR adds first-class support for ThunderID as a built-in OpenID Connect provider (packages/core/src/providers/thunderid.ts).

Also includes:

• Provider docs page (docs/pages/getting-started/providers/thunderid.mdx)
• SVG logo (docs/public/img/providers/thunderid.svg)
• Docs manifest entry
• Dev app env example entries (AUTH_THUNDERID_ID, AUTH_THUNDERID_SECRET, AUTH_THUNDERID_ISSUER)

🧢 Checklist

• Documentation
• Tests
• Ready to be merged

🎫 Affected issues

• [Provider] Add ThunderID as a built-in Auth.js provider #13462

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

[vercel]

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Project	Deployment	Actions	Updated (UTC)
auth-docs	Ready	Preview, Comment	Jun 25, 2026 4:22pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
next-auth-docs	Ignored	Preview	Jun 25, 2026 4:22pm

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.93%. Comparing base (7b35fc3) to head (0a629d1).

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #13460       +/-   ##
===========================================
+ Coverage   49.85%   93.93%   +44.08%     
===========================================
  Files         200        4      -196     
  Lines       13487      462    -13025     
  Branches     1445       76     -1369     
===========================================
- Hits         6724      434     -6290     
+ Misses       6763       28     -6735     

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[brionmario]

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[brionmario]

Hey @Bekacru,

Would you mind taking a look at this PR when you have a chance? We'd really appreciate it if you could let us know whether it's good to merge.

For some context, we've also submitted a PR to better-auth to have ThunderID added as a provider. However, we still have many customers who haven't migrated yet, so it would be great if we could get ThunderID listed in Auth.js as well.

Thanks!

Regards,
Brion