fix: make shared modal accessible

[byot-dot]

Improves keyboard and assistive-technology support for the shared Modal component without changing its visual layout.

• adds dialog semantics with role="dialog" and aria-modal
• associates the dialog with its visible title and subtitle
• gives the icon-only close button an accessible name
• moves focus into the dialog when it opens and restores focus on close
• contains Tab / Shift+Tab focus within the dialog
• keeps Escape dismissal scoped to the active dialog

The change is limited to src/components/UI/Modal/Modal.tsx.