Use jpg avatar placeholder in demo partials ...

Switch all demo avatar references from /assets/avatar-placeholder.png
to /assets/avatar-placeholder.jpg as requested.

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

polish: use icon-only buttons in navigation-shell footer demo

feat: add remaining reference Vue components, card variants, toolbar/repeater/activity-log/usage-meter styles and demo mounts ...

- Add GnHorizontalCard, GnUserCard, GnMetricCard, GnStatusCard, GnLoginCard,
  GnIdentity, GnActivityLog, GnToolbar, GnRouterTabs, GnRepeater, GnUsageMeter
- Extend GnTabs to support reference items API while keeping child mode
- Add card subcomponent styles: status, metric, horizontal, login, user
- Add toolbar, repeater, identity, activity-log, usage-meter styles
- Add typography token variables used by reference-compatible components
- Add Vue demo mounts for cards, data-patterns, progress, tabs, navigation-shell
- Fix GnAvatar online status class and navigation-shell footer slot
- Update screenshot config to cover skeleton and navigation-shell

Verified: npm run build, npm run test:e2e (19 passed), npm run screenshots,
npm run test:vue-adapter, figma-match (7 passed)

Add Table, List, Skeleton, Tooltip, Dropdown, NavigationShell ...

- Implement Table styles + browser helper + GnTable/GnTableRow/GnTableHeader
  with sortable/selectable variants.
- Implement List styles + browser helper + GnList/GnListItem.
- Implement Skeleton placeholders + browser helper + GnSkeleton.
- Implement Tooltip styles + browser helper + GnTooltip.
- Implement Dropdown menu styles + browser helper + GnDropdown/GnDropdownItem.
- Implement NavigationShell layout + browser helper + GnNavigationShell/GnNavigationItem.
- Wire all new components into kit.scss, Vue plugin, Vue entry, and browser bundle.
- Add demo partials for every new component.
- Update docs/component-coverage.md and PROJECT_NOTES.md.
- Builds and tests pass.