zainul991/NexaAI ? reverse-engineered prompt

Build me a ChatGPT style web app called Nexa with a clean, modern chat interface. I want users to be able to start new conversations, switch between past chats, and delete threads from a sidebar. The chat history should be saved in MongoDB so it stays there after refresh, and the assistant responses should come from the OpenAI API.

Make the messages render nicely with markdown, including code blocks with syntax highlighting, and add a simple typing animation when the assistant replies so it feels alive. Keep the UI minimal and polished, similar to ChatGPT. Use a React frontend and a Node and Express backend with MongoDB on the server side.

Please wire up the full flow so sending a message saves it, gets a reply from OpenAI, and shows both in the current thread. If you need current setup details for the API or any library behavior, look up the latest docs online and use sensible defaults.