React-powered Responsive Newsletter Form

While working on this project, I found it a bit tricky to manage things with React, especially handling the transition between the main page and the success message.

Since it's my first time using useState, I'm not completely sure about how to use React hooks like useState and useEffect.

I'm wondering if there are some good ways to organize components, manage state, and handle component lifecycle methods that I might have missed. Any advice or feedback is welcome!