🎨 Palette: Improve background overlay accessibility in modals

[ford442]

💡 What: Standardized background overlay implementation for modale dialogs.
🎯 Why: Having onClick bound to the main wrapper or an overlay without aria-hidden="true" causes screen readers to pick up the entire background as an interactive element.
📸 Before/After: Visuals remain unchanged, but the DOM structure ensures the click handler is attached to a hidden sibling of the dialog rather than its parent wrapper.
♿ Accessibility: Ensures assistive technologies ignore the modal backdrop overlay while maintaining its click-to-dismiss behavior.

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PR created automatically by Jules for task 6973499334968866107 started by @ford442

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