🎨 Palette: Accessibility improvements for sidebar action buttons

[thirdeyenation]

💡 What: Added aria-label attributes to the icon-only action buttons in the sidebar Chat List and Task List components, matching their existing title tooltips. Added aria-hidden="true" to the inner <span class="material-symbols-outlined"> elements.

🎯 Why: Improves accessibility for screen reader users by providing descriptive text for interactive elements instead of relying solely on visual icons or having the screen reader announce raw ligature text (like "delete sweep").

📸 Before/After: Visually identical. A11y tree now correctly announces "Delete task" instead of "close button".

♿ Accessibility:

• Added aria-label to buttons without visible text.
• Added aria-hidden="true" to material symbol spans to prevent screen readers from reading the raw ligature text.

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PR created automatically by Jules for task 11463914756701170978 started by @thirdeyenation

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