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>

Add GnAvatarStack, GnChipGroup, GnCombobox and GnSearchField Vue components ...

- Mirror reference gnexus-ui-kit API for avatar stack, chip group, combobox and search field.

- Add supporting styles for .avatar-stack, .avatar-xs and .chip-group.

- Add Vue demo mounts to avatar, chips, advanced-select and inputs partials.

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

Add Vue components and demo mounts for divider, step, chips, avatar, choices, select, textarea, pagination, loader ...

- Create GnDivider and GnStep Vue components.
- Export GnStep and GnDivider via plugin and index.
- Update GnSteps to render check icon for complete steps.
- Add Vue demo mounts to divider, stepper, select-textarea, pagination, loader, chips, avatar, choices partials.
- Convert chips and avatar partials to demo-section for screenshot capture.
- Expand screenshot section list.
- All 19 e2e tests green.

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

Add Tabs, Avatar, Chip, and Progress components ...

- Implement pill/compact/vertical tabs with browser helper and Vue adapters
  (GnTabs, GnTab, GnTabPanel) plus keyboard navigation.
- Add Avatar styles and GnAvatar with initials/image/icon and status dot.
- Add Chip styles and GnChip removable/selectable chip.
- Add Progress styles, browser helper, and GnProgress linear/step.
- Wire new components into kit.scss, Vue plugin, Vue entry, and browser bundle.
- Add demo partials and update docs/component-coverage.md + PROJECT_NOTES.md.
- All builds and smoke tests pass.