🎨 Palette: Improve accessibility of chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label to the chat navigation buttons and aria-hidden="true" to their inner material symbol spans.
🎯 Why: Screen readers would previously read out the ligature text (like "vertical_align_top") which is confusing for users. Now they will read clear, descriptive labels.
📸 Before/After: N/A - visual appearance remains unchanged.
♿ Accessibility: Improved screen reader compatibility for icon-only navigation buttons.

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PR created automatically by Jules for task 18153643285080470164 started by @thirdeyenation

[google-labs-jules]

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