Verify the chat app works at {{URL}}.

## Steps

1. Open {{URL}} with agent-browser
2. Take a snapshot to find the message input and send button
3. Type a test message like "Hello world" into the input
4. Click the send button
5. Wait briefly for the state to update
6. Take a new snapshot and check the message appears in the chat

## Pass criteria

- The page loads without errors
- There is a text input for typing messages
- There is a send button
- After sending a message, it appears in the message list
