Add Kakao (OIDC) as an SSO provider or make Manual OIDC truly generic

We need Kakao (OIDC) as an SSO option for Korea-facing apps.

The current “Advanced / Manual Configuration” still fails at callback with 401 because it assumes Google/MS/Okta/GitHub conventions.

Please add either:

• A first-class Kakao provider, or

• Generic OIDC support with:

1. Claim mapping (email claim path like kakao_account.email)

2. PKCE end-to-end (code_verifier at token exchange)

3. Flexible scope and required-claim checks

4. Strict Discovery/JWKS handling compatible with Kakao