🎨 Palette: Add ARIA labels to chat navigation buttons

[thirdeyenation]

💡 What

Added aria-label to the icon-only chat navigation buttons (scroll to top/bottom, previous/next user message) and aria-hidden="true" to their inner material symbol spans.

🎯 Why

Screen readers were previously unable to announce the purpose of these icon-only buttons, and might read the raw ligature text (e.g. "vertical align top") instead, leading to a confusing accessibility experience.

📸 Before/After

No visual changes. Code only.

♿ Accessibility

Significantly improves screen reader navigation in the main chat area by providing clear, descriptive labels for chat navigation controls while hiding the decorative icon spans.

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PR created automatically by Jules for task 18429417885196953939 started by @thirdeyenation

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