fix(a11y): improve color contrast ratios for WCAG AA compliance

[huangkevin-apr]

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Changelog

Fixed multiple color contrast violations detected by IBM Equal Access Accessibility Checker to meet WCAG AA requirements:

• Introduced --tw-color-blue-primary-dark (#004a8d) for improved contrast ratio (≥4.5:1 for white text)
• Applied darker blue background to nav, header, and footer elements
• Increased font size of .home-link to meet large text contrast requirements (≥3.0:1)
• Updated Twitter timeline embed link colors to use the darker blue for sufficient contrast

This resolves 20 accessibility violations related to text contrast ratios across the homepage.

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Screenshots

Before:
Multiple WCAG AA contrast violations on light blue (#1DA1F2) background - contrast ratio ~3.0:1 with white text

After:
All text elements now meet WCAG AA requirements with darker blue (#004a8d) background - contrast ratio ≥4.5:1 for normal text, ≥3.0:1 for large text

• Note: The patch submitted in this PR was generated by A11YRepair, an automated Web Accessibility repair tool that I developed to address common accessibility violations in web applications. The generated fixes were manually reviewed and validated before submission.

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