Refactor buttons to semantic anchor tags for accessibility

[tonymendonca23]

• What: Replaced div.btn > h4 and div.sign-up-btn > h4 with <a href="#" class="...">.
• Why: To improve accessibility by making the "Join Us" and "Sign up" buttons focusable and semantically correct for screen readers.
• Visuals: Maintained visual consistency by adding font-weight: 700 and font-family: inherit to the CSS classes.
• Verification: Verified with Playwright (checks for tag name and focusability) and visual inspection.

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PR created automatically by Jules for task 458192410125905560 started by @tonymendonca23

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