🎨 Palette: Add aria-labels to sidebar icon-only buttons for accessibility

[thirdeyenation]

💡 What: Added explicit aria-label attributes to the icon-only action buttons in the chat and task sidebars (Close chat, View task details, Clear task chat, Delete task).
🎯 Why: These buttons previously only used the title attribute, which is not reliably announced by all screen readers. Explicit aria-labels ensure consistent accessibility.
📸 Before/After: Visuals remain unchanged. Screen readers now properly announce the button actions instead of reading generic elements.
♿ Accessibility: Greatly improves screen reader navigation and confidence when managing chats and tasks in the sidebar.

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PR created automatically by Jules for task 18040098044063398385 started by @thirdeyenation

[google-labs-jules]

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