🎨 Palette: Improve GlobalSearchModal accessibility

[criptogus]

💡 What:

• Converted GlobalSearchModal search results from clickable divs to <button> elements.
• Added explicit focus-visible styles for better keyboard navigation visibility.
• Added <DialogDescription> to the modal for screen reader context and to resolve Radix UI warnings.

🎯 Why:

• Search results were previously inaccessible to keyboard users (could not tab to them) and screen readers (announced as generic groups).
• The modal was triggering accessibility warnings due to a missing description.

♿ Accessibility:

• Keyboard: Users can now Tab through search results and select them with Enter/Space. Focus rings are now clearly visible.
• Screen Readers: Results are announced as buttons. The modal purpose is now described via DialogDescription.

Testing:

• Verified with pnpm test src/tests/components/GlobalSearchModal.test.tsx (new test added).
• Visually verified focus states via screenshot.

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PR created automatically by Jules for task 13938643083951283489 started by @criptogus

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