🎨 Palette: Replace div buttons with semantic <button> elements

[google-labs-jules]

💡 What: Replaced accessibility-hostile div buttons with semantic <button> elements in index.html and updated style.css to maintain visual parity.

🎯 Why: To improve accessibility for keyboard and screen reader users. The previous implementation used div tags which are not focusable or interactive by default.

📸 Verification: verified with Playwright that elements are now BUTTON tags and visually identical.

♿ Accessibility:

• Replaced div with <button>.
• Removed inner h4 tags (semantically incorrect inside buttons).
• Added font-weight: 700 and font-family: inherit to CSS to restore visual design.

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PR created automatically by Jules for task 3664414155192215932 started by @tonymendonca23

[google-labs-jules]

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