🎨 Palette: [UX improvement] Add accessibility labels to chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label to chat navigation buttons and aria-hidden="true" to their internal icons.
🎯 Why: Screen readers would previously either read nothing or read the raw material symbol ligature text (e.g., "vertical align top") instead of the intended button action, making the chat navigation difficult to use for visually impaired users.
📸 Before/After: N/A (Non-visual HTML attribute change)
♿ Accessibility: Enhances screen reader support by providing explicit aria-labels that match tooltips and hiding the raw icon ligature text from assistive technologies.

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PR created automatically by Jules for task 7533661451543013739 started by @thirdeyenation

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