🎨 Palette: Add ARIA labels to chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label attributes to the chat navigation buttons (scroll to top/bottom, previous/next user message) and added aria-hidden="true" to their inner material symbol spans.
🎯 Why: These are icon-only buttons that rely on title attributes for tooltips, but screen readers and assistive technologies need explicit aria-labels to announce their purpose, while hiding the raw ligature text.
📸 Before/After: No visual changes.
♿ Accessibility: Improves keyboard navigation and screen reader support for the chat interaction tools.

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PR created automatically by Jules for task 17259503425059100097 started by @thirdeyenation

[google-labs-jules]

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