Add aria-modal to improve a11y

[Schleuse]

Add's aria-modal="true" on the dialog element.

A section of content is "modal" means navigation is limited to the area itself and the background (the ancestors and siblings of the modal) is hidden. Setting aria-modal="true" on dialog and alertdialog role containers indicates the presence of a "modal" element to users of assistive technology, but does not actually make the element modal. The features that make the element actually modal must be implemented by the developer.

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-modal