Add GnConfirmDialog and confirmPopup helper ...

- Create GnConfirmDialog Vue component wrapping GnModal
- Add browser Modals helper and confirmPopup for gnexus-ui-kit compatibility
- Wire exports in src/js/index.js, src/vue/index.js and plugin
- Add demo partial for confirm dialog
- Build and e2e tests pass (17/17)

Co-Authored-By: Claude <noreply@anthropic.com>