Adds "Login with <provider>" as an alternative login method using the
jumbojett/openid-connect-php library. OIDC users must already exist in
the database (matched by email). Configurable via OIDC_PROVIDER_URL,
OIDC_CLIENT_ID, OIDC_CLIENT_SECRET, and OIDC_PROVIDER_NAME constants.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
