Update app.py

feat: Add a demo video link to the UI and enhance styling for the workflow overview, main cards, and overall page container.

feat: enable and disable Gradio buttons during security tests and tutor interactions

Remove `share=True` argument from Gradio interface launch.

chore: Remove explicit Gradio launch parameters for server port, share, and inbrowser.

feat: implement VAPT agent application with a Gradio UI, AI tutor, and dashboard, and update README.