Add overlay components: Modal, Drawer, Toast system ...

- Add SCSS for modals, drawers and toasts with VMK design tokens.
- Add GnModal and GnDrawer Vue adapters with focus trap, Escape close and teleport.
- Add browser Toasts helper and GnToastProvider + useToast composable.
- Wire overlays into plugin, Vue entry and browser bundle.
- Add demo partials for Modal, Drawer and Toast.
- Update component coverage and project notes.

Add Input, Badge, Alert components and foundation demos ...

- Add src/scss/components/_forms.scss with input, label, help text,
  and error/success/warning states.
- Add src/scss/components/_badges.scss and _alerts.scss with semantic
  variants and outline/size options.
- Add GnInput, GnBadge, GnAlert Vue adapters mirroring gnexus-ui-kit API.
- Update plugin and entry exports for new components.
- Add demo partials for Typography, Palette, Utilities, Inputs, Badges,
  and Alerts, and include them in demo/index.html.
- Extend Vue example app to render new components.

Implement Button component and Vue adapter ...

- Add src/scss/components/_buttons.scss with Figma-extracted variants,
  sizes (lg/md/sm/xs/xxs), states (disabled, loading), icon support,
  and destructive secondary/tertiary styles.
- Add src/vue/components/GnButton.js and VmkIcon with inline SVG loading.
- Add shared src/vue/utils.js for class/variant/size normalization and
  legacy Phosphor-name mapping.
- Wire GnButton and VmkIcon into the Vue plugin and entry exports.
- Add demo partial for buttons and update demo index page.