🎨 Palette: Improve Dialog accessibility

[ivanleekk]

💡 What: Added standard accessibility (a11y) attributes (aria-hidden, role, aria-modal, and aria-label) to the Dialog component.
🎯 Why: To make custom modals natively accessible to screen readers, ensuring the backdrop is ignored, the modal container is properly recognized, and the close button is explicitly labeled.
📸 Before/After: No visual changes.
♿ Accessibility: Added aria-hidden="true" to the dialog backdrop. Added role="dialog" and aria-modal="true" to the main modal container. Added aria-label="Close" to the icon-only close button.

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PR created automatically by Jules for task 15118380983164708432 started by @ivanleekk

[google-labs-jules]

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