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.