Build a simple chat room application backed by a RivetKit actor.

Requirements:
- A single chat room actor that stores messages in state
- A `sendMessage` action that adds a message to the chat
- Messages should have at least a text field and a timestamp
- A React frontend that displays the message list and has an input field with a send button
- Use Vite for the dev server
- Use the file-system driver (standalone mode, no external server)
- The app should run with `npm run dev` on port 5173
