Enhancement: Add 'Clear Chat' Button

[adarsh832]

We need a way for users to easily clear their current conversation and start fresh without restarting the app.

Task:

1. Add a small 'Trash' or 'Clear' icon button in the top bar of \lib/screens/chat/chat_screen.dart.
2. When tapped, it should show a simple confirmation dialog ('Are you sure you want to clear the chat?').
3. If confirmed, reset the chat state (clear the messages list).

This is a great issue for someone new to Flutter or this project! It involves basic UI (Icons, Dialogs) and simple state management.