🎨 Palette: Refactor buttons to semantic HTML

[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 div implementation was not focusable or interactive.
📸 Before/After: Visually identical, but now focusable with a high-contrast outline.
♿ Accessibility: Added role="button" (native), keyboard focus support, and :focus-visible styles using var(--main-text) for visibility on dark/blue backgrounds. Also fixed a legacy CSS layout issue in .call-content.

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PR created automatically by Jules for task 15302337262215199683 started by @tonymendonca23

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