🎨 Palette: [UX improvement] Improve chat navigation button accessibility

[thirdeyenation]

"💡 What: Added aria-label to the chat navigation buttons and aria-hidden='true' to the inner span containing the Material Symbols ligatures.
🎯 Why: Screen readers could confusingly read the raw ligature text (like 'vertical_align_top') without context, making navigation unclear.
📸 Before/After: No visual change; strictly an accessibility improvement.
♿ Accessibility: Improved screen reader support by ensuring the buttons have clear labels and the confusing raw icon text is hidden from assistive technologies."

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PR created automatically by Jules for task 4559638298111643186 started by @thirdeyenation

[google-labs-jules]

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