Build a single-page app using React (frontend) and Node.js + Express (backend). The app allows a user to input a YouTube URL and click "Generate Quiz". The backend fetches the video transcript, sends it to OpenAI GPT-4 to generate 5 multiple-choice questions with 4 options, correct answers, explanations, and timestamps. The frontend shows the embedded YouTube video (use react-player), displays the quiz, and lets the user submit answers. The backend validates answers, returns a score, and shows explanations with timestamps. Users can click a timestamp to jump to that point in the video. No login or authentication is needed. Use Tailwind CSS or Material UI for styling. Use axios for HTTP and youtube-transcript npm package to fetch transcripts.