🎨 Palette: Improve Dialog accessibility

[ivanleekk]

💡 What: Added standard WAI-ARIA roles and attributes to the shared Dialog component.
🎯 Why: To ensure screen readers properly interpret the modal container as a dialog, prevent the background overlay and graphical X icon from creating noise, and provide an explicit text label for the icon-only close button.
📸 Before/After: The visual appearance remains completely unchanged.
♿ Accessibility:

• Added role="dialog" and aria-modal="true" to the inner modal container.
• Added aria-hidden="true" to the decorative backdrop overlay.
• Added aria-label="Close dialog" to the close <button>.
• Added aria-hidden="true" to the purely decorative <X> SVG icon.

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PR created automatically by Jules for task 4740460228249345061 started by @ivanleekk

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