🎨 Palette: Add ARIA labels to app shell icon buttons

[artosien]

This PR implements a micro-UX accessibility improvement.

💡 What: Added explicit aria-label attributes to the icon-only buttons in the main navigation header and slide-out sheets within app-shell.tsx.
🎯 Why: Icon-only buttons (using size="icon" from shadcn) lack implicit text context for screen readers. By adding aria-label="Open alerts", "Open menu", and "Close tickets", we ensure users navigating via keyboard or assistive technology understand the function of these actions.
📸 Before/After: Visuals remain unchanged.
♿ Accessibility: Improves WCAG compliance for navigation elements.

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PR created automatically by Jules for task 11632022109421065944 started by @artosien

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