🎨 Palette: Convert Div Buttons to Semantic HTML

[tonymendonca23]

💡 What: Refactored "Join Us" and "Sign up" buttons from <div> to <button> elements and fixed invalid CSS syntax in .call-content.
🎯 Why: Legacy div buttons were inaccessible to keyboard users and screen readers.
📸 Before/After: Visual typography is preserved using explicit inheritance; new keyboard focus rings added.
♿ Accessibility: Replaced non-semantic tags with <button>, added :focus-visible styles, and ensured tab focus works.

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PR created automatically by Jules for task 16531412916128158173 started by @tonymendonca23

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