Add simple button page

[ylee-figma]

Summary

Adds a simple, self-contained index.html with a styled button.

Details

• Centered button using flexbox
• Blue button with hover, active (press), and focus-visible states for accessibility
• Clicking shows an alert (Button clicked!)
• No dependencies — just open index.html in a browser

Testing

Open index.html in any modern browser and click the button.