fix: close high-severity compatibility gaps from functional audit ...

- Add missing Drawer browser helper (src/js/components/drawer.js) and export it.
- Replace icon-only Helper with full gnexus-ui-kit-compatible Helper object
  (template, unification, states).
- Make Toasts create* methods synchronous to match the reference contract.
- Rename Vue plugin export to GnexusUiVue while keeping VMKUiVue alias;
  add tryUseRouter, tryUseRoute, isRouteActive composables.
- Emit both vmk-ui-kit.* and gnexus-ui-kit.* browser bundles; point package.json
  main/module/exports to gnexus-ui-kit aliases.
- Add demo partials for GnInputGroup, GnDescriptionList, GnEmptyState, GnPageHeader.
- Update modal/drawer/toast demos to exercise the browser JS API.
- Update functional tests for dynamic modal/drawer creation.

All e2e tests pass; demo page has no console errors with the new background wrapper.

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