- Add Content-Security-Policy to main nginx server block (was only on /admin/)
- Add Cross-Origin-Opener-Policy and Cross-Origin-Resource-Policy headers
- Add includeSubDomains to HSTS header
- Set HttpOnly, Secure, SameSite=Lax session cookie params on public pages
(AdminAuth already hardens the /admin session with SameSite=Strict)
- Update xamxam.conf.reference and SECURITY_HEADERS.md to match