/sso/fiscal now requires the state that GTI Fiscal creates when the flow
starts and signs it as the "nonce" claim; GTI Fiscal checks it against
its own one-time cookie. Without a valid state the panel sends the
browser to GTI Fiscal's /login, so the flow always starts there. This
prevents someone from pushing an assertion for their own account into
another person's browser (login CSRF).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>