🎨 Palette: Improve accessibility and semantic navigation

[ISTEC-FOR340]

This PR implements several micro-UX improvements focused on accessibility and semantic correctness:

• Semantic Navigation: Replaced div containers with onclick listeners with semantic <a> tags for cards and "Back" buttons. This restores native browser behaviors like opening in new tabs and improves screen reader support.
• Keyboard Accessibility: Added :focus-visible styles to all interactive elements (cards, buttons, sliders, inputs) to ensure a clear focus indicator for keyboard users.
• Accessibility Metadata: Updated/added lang="en" to HTML files and added aria-label to navigation links.
• Cleanup: Removed redundant script.js and associated <script> tags as navigation is now handled by semantic HTML.
• Hygiene: Normalized line endings to LF for modified files.

These changes ensure the application is more robust, accessible, and follows web standards while maintaining the original visual design.

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PR created automatically by Jules for task 17020946049442593697 started by @ISTEC-FOR340

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