🎨 Palette: Improve keyboard accessibility for scrollable regions

[n24q02m]

💡 What: Added explicitly high contrast :focus-visible outlines to navigation links, buttons, theme toggle, and the table scroller. Also explicitly made the table scroller keyboard focusable using tabindex="0", role="region", and an aria-label.
🎯 Why: Without an explicit tabindex, keyboard users cannot scroll overflow tables on small screens. Standard button focus states that only rely on a 1.08x brightness increase are nearly invisible and fail WCAG contrast requirements, preventing keyboard users from seeing what element is active.
📸 Before/After: Before, buttons barely changed brightness when focused. Now, interactive elements have a clear, distinct 2px accent outline.
♿ Accessibility: Ensures keyboard scrollability on overflow containers and clear focus indications across the UI.

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PR created automatically by Jules for task 4370215155702426918 started by @n24q02m

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