feat: add option to disable local login

[aruznieto]

Summary

• add OVERLEAF_DISABLE_LOCAL_LOGIN to hide the local login form when external auth is enabled
• keep configured LDAP/SAML/OIDC login buttons visible
• block direct POST /login attempts when local login is disabled
• include OIDC in external-auth feature detection and add unit coverage

Testing

• git diff --check
• unit test command could not be completed locally because the workspace lockfile is missing @overleaf/linked-url-proxy@workspace:services/linked-url-proxy

[aruznieto]

What's happens with that? @Musicminion