Add Button UI Lab project (Frontend + Server setup)

## Goal: Add the Button UI Lab project files into the repository.

Tasks

 Add project structure (public/, server.js, package.json)

 Add .gitignore for Node.js

 Verify app runs locally using node server.js

 Push changes using feature branch + Pull Request

 Get 1 approval and merge PR into main

Expected Output

Button UI Lab should run successfully at:
http://localhost:3000