🎨 Palette: Improve keyboard accessibility and semantic navigation

[ISTEC-FOR340]

I have improved the keyboard accessibility and semantic structure of the 'MySketches' project.

Key changes include:

• Skip to main content: Added a skip link to allow keyboard users to bypass the header.
• Semantic Navigation: Replaced div cards on the landing page and button elements on project pages with semantic <a> tags. This ensures navigation works without JS and leverages native browser features.
• Redundancy Cleanup: Removed the onclick listeners in script.js as they are no longer needed.
• Visual Focus Indicators: Implemented :focus-visible styles in CSS to provide high-contrast focus rings for keyboard users while maintaining a clean aesthetic for mouse users.
• A11y Refinements: Ensured that link text is properly announced by screen readers by removing unnecessary aria-hidden attributes on call-to-action spans.

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PR created automatically by Jules for task 8176865489169789706 started by @ISTEC-FOR340

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