Validates, sanitizes, and stores a redirect URL in a secure cookie.

This function prevents open redirect and cookie injection vulnerabilities by:

• Removing dangerous characters (;, \r, \n) that could enable header/cookie injection
• Enforcing relative URLs only (must start with /) to prevent external redirects
• Encoding the URL for safe cookie storage
• Setting appropriate cookie security flags (HttpOnly)

The redirect URL is extracted from the request URL, validated, and stored in a cookie with a 60-second TTL for use after authentication flows.