🎨 Palette: Improve keyboard accessibility for scrollable table and interactive elements

[n24q02m]

💡 What:
Added a tabindex="0", role="region", and an aria-label to the table wrapper (.scroller) so that keyboard users can scroll and navigate it. I also updated the :focus-visible pseudo-class for all interactive elements (nav a, .theme-toggle, .btn, and .scroller) to use a highly visible outline with outline-offset, rather than relying solely on subtle brightness filters.

🎯 Why:
Previously, the table container did not receive focus, preventing keyboard-only users from scrolling it if it overflowed horizontally on smaller screens. Additionally, the focus states on interactive elements were inadequate for visually identifying the active element, as a simple brightness change does not meet WCAG contrast guidelines for focus indicators.

📸 Before/After:
(Visual changes verified using Playwright screenshots, showing distinct outlines on focused elements).

♿ Accessibility:

• Resolved a WCAG issue where horizontal scroll containers were inaccessible via keyboard.
• Replaced non-compliant brightness-based focus states with robust, high-contrast outlines for all main interactive elements.

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PR created automatically by Jules for task 12286923531298649205 started by @n24q02m

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