Navigating from login reused a send_email_code token on register.
Captcha failure used 401 so the client stored /register as the
post-login target and never left the page. Permission denials now
return 403, and the API client no longer wipes the session on 401.
Sanitize and validate redirect targets from callbackUrl parameter and sessionStorage in login, registration, and OAuth callback flows.
Introduced safeRedirectTarget helper which rejects protocol-relative URLs, non-relative schemes, control characters, backslashes, and encoding bypasses.
ryan