🎨 Palette: Refactor div buttons to semantic HTML

[google-labs-jules]

💡 What: Converted the "Join Us" and "Sign up" buttons from <div> elements to semantic <button> elements. Removed inner <h4> tags and moved typography styles to CSS.
🎯 Why: Improved accessibility for keyboard users and screen readers. The buttons are now focusable and announce correctly.
📸 Before/After: Visual appearance is identical.
♿ Accessibility: Added :focus-visible styles with high contrast outlines (using --main-text on dark/brand backgrounds) to ensure keyboard focus is clearly visible.
Refactored CSS to fix legacy flex-direction error in .call-content.

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PR created automatically by Jules for task 11104776588386677680 started by @tonymendonca23

[google-labs-jules]

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