🎨 Palette: Semantic and accessible navigation

[ISTEC-FOR340]

💡 What: Refactored navigation cards and back buttons from non-semantic div/button elements with manual JavaScript handlers to semantic tags.

🎯 Why: This change restores native browser behaviors (like middle-click to open in a new tab), simplifies the codebase by removing manual location handling, and significantly improves accessibility for keyboard and screen reader users.

♿ Accessibility:

• Added high-contrast :focus-visible indicators for all interactive cards and buttons.
• Ensured valid HTML by converting nested interactive elements (buttons) to descriptive spans within the parent link.
• Verified keyboard tab order and focus states across the landing page and subprojects.
• Set proper language attributes in subprojects for screen reader consistency.

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PR created automatically by Jules for task 17650529940351359650 started by @ISTEC-FOR340

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