🎨 Palette: [UX improvement] Add ARIA labels to icon-only buttons for accessibility

[thirdeyenation]

💡 What: Added aria-label attributes to icon-only buttons across the chat sidebar, task sidebar, and chat navigation. Also added aria-hidden="true" to their inner Material icon <span> elements.
🎯 Why: To improve screen reader accessibility. Without aria-label, screen readers might read the literal ligature text (e.g., "keyboard_arrow_up") or announce an unlabeled button, making the UI inaccessible to visually impaired users.
📸 Before/After: Visuals are unchanged as this is a DOM attribute accessibility improvement. Verified via DOM inspection in Playwright.
♿ Accessibility: Ensures all icon-only action buttons are properly announced by assistive technologies and hides the visual-only ligature icons from screen reader parsing.

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PR created automatically by Jules for task 12071580801479507774 started by @thirdeyenation

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