🎨 Palette: Improve button accessibility

[tonymendonca23]

💡 What: Refactored "Join Us" and "Sign up" buttons from <div> wrappers to semantic <button> elements.
🎯 Why: To improve accessibility for keyboard users and screen readers. The previous implementation was not focusable or interactive.
📸 Before/After: Visual appearance is preserved, but buttons now have a visible focus state (outline) and are keyboard accessible.
♿ Accessibility:

• Replaced div with <button>.
• Added :focus-visible styles with sufficient contrast.
• Ensured keyboard navigation works.

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PR created automatically by Jules for task 10450793552486316051 started by @tonymendonca23

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