🎨 Palette: Improve Dialog accessibility

[ivanleekk]

💡 What:
Added critical ARIA attributes to the custom Dialog component in frontend/src/components/ui/Dialog.tsx.

• Added role="dialog" and aria-modal="true" to the modal container.
• Added aria-hidden="true" to the backdrop to hide it from screen readers.
• Added aria-label="Close dialog" to the icon-only close <button>.

🎯 Why:
Custom modal components require specific ARIA attributes to be properly announced and interacted with by screen readers. Without these, visually impaired users may not realize a modal has opened, may get lost navigating the background content, or won't know what the icon-only close button does.

📸 Before/After:
(Visuals remain unchanged, this is an underlying HTML structure improvement)

♿ Accessibility:
This change significantly improves the accessibility of all modals across the application for users relying on assistive technologies, ensuring they comply with WAI-ARIA authoring practices for dialogs.

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PR created automatically by Jules for task 11781407993200268815 started by @ivanleekk

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