Refactor buttons to semantic HTML for accessibility

[tonymendonca23]

🎨 Palette: Improved accessibility of buttons

💡 What:

• Converted "Join Us" and "Sign up" buttons from <div> wrappers to semantic <button> elements.
• Removed inner <h4> tags and placed text directly in buttons.
• Updated CSS to reset button styles (font, border, background) to match original design.
• Added :focus-visible styles for keyboard navigation.
• Fixed invalid flex-direction: space-between in .call-content to justify-content: space-between.

🎯 Why:

• Previous div buttons were inaccessible to keyboard users and screen readers.
• Focus states improve navigation for power users and those with disabilities.
• Layout bug fix ensures proper spacing in the call-to-action section.

📸 Before/After:

• Visually identical (verified via screenshots), but now accessible.

♿ Accessibility:

• Added semantic roles (button).
• Added keyboard focus support.
• Improved contrast for focus indicators.

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PR created automatically by Jules for task 2470182584490622225 started by @tonymendonca23

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