Redmine Agile plugin demo

Build a reusable JavaScript method to perform an AJAX POST request using fetch, sending JSON payloads and handling CSRF tokens when required. The method should validate inputs, set correct headers, and serialize/deserialize JSON safely. It must implement robust error handling for non-2xx responses, network failures, and timeouts, returning a clear, Promise-based result. Include concise documentation and examples so it can be dropped into any project with minimal setup.