a11y: ModalDialog does not set aria-modal="true" on the dialog element #4402

Context

Found during the 2026 Axim-commissioned accessibility audit of Paragon, while documenting ModalDialog against the APG modal dialog pattern.

Current behavior

ModalDialog renders the dialog container with role="dialog" and aria-label={title} (ModalDialog.tsx), and ModalLayer enforces focus containment behaviorally via react-focus-on. However, aria-modal="true" is not set.

Impact

Some screen readers use aria-modal to scope reading/browse commands to the dialog and treat background content as inert. Without it, users may be able to read (though not focus) background content while the dialog is open, contrary to the modal's intent. WCAG mapping: 4.1.2 Name, Role, Value (state not fully exposed).

Suggested fix

Add aria-modal="true" to the role="dialog" element in ModalDialog.tsx. Verify the derived modals (AlertModal, FullscreenModal, MarketingModal) inherit it, and confirm no double-inert conflict with the react-focus-on aria-hidden handling.

References

• APG modal dialog pattern: https://www.w3.org/WAI/ARIA/apg/patterns/dialog-modal/
• Related ACT rule: Element with role attribute has required states and properties