🎨 Palette: Added ARIA labels to icon-only buttons

[artosien]

💡 What: Added aria-label attributes to the Bell, Menu, and Close icon-only buttons in the AppShell component.
🎯 Why: To improve accessibility for screen reader users by providing clear context for these controls, which previously had no readable text.
♿ Accessibility: Ensures that users relying on assistive technologies can understand the function of the notification, menu, and close buttons.

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PR created automatically by Jules for task 9979556811409661278 started by @artosien

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