🎨 Palette: Semantic buttons & focus states

[tonymendonca23]

Refactored "Join Us" and "Sign up" buttons from non-semantic <div> elements to <a> tags. This change enables keyboard accessibility (tab focus) and provides correct semantic roles for screen readers.

Key changes:

• Replaced <div class="btn"><h4>...</h4></div> with <a class="btn">...</a>.
• Added font-weight: 700 to .btn and .sign-up-btn classes to replicate the visual weight of the removed <h4> tags.
• Added :focus-visible outline styles to ensure keyboard users can track focus.
• Verified visual parity and focus functionality using Playwright.

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PR created automatically by Jules for task 5808225050421755602 started by @tonymendonca23

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