Fix Phosphor icon class on GnButton/GnIconButton usages. ...

gnexus-ui-kit's iconNode() does not prepend the base `ph` class
when the icon string starts with `ph-`; it only keeps the string
as-is. All icon props must include the base class explicitly:
`icon="ph ph-sidebar"` instead of `icon="ph-sidebar"`.

Updated 15 icon bindings across 9 components.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Wire gnexus-ui-kit v0.2.0 Vue 3 adapter and migrate Wave 1 components. ...

- Update vendor/gnexus-ui-kit to v0.2.0 (Vue components + composables)
- Add Vite aliases for gnexus-ui-kit/vue and gnexus-ui-kit/css
- Install GnexusUiVue plugin in main.js + GnToastProvider in App.vue
- Migrate ProfileBadge → GnBadge
- Migrate ErrorMessage → GnAlert
- Migrate ConfirmDialog → GnConfirmDialog
- Migrate raw <button> elements → GnButton / GnIconButton in:
  WelcomeScreen, LoginScreen, ChatArea, ChatHeader, AppSidebar,
  ArtifactsPanel, UserMessage, AssistantMessage, SelectionToolbar

Build and tests pass (51/51).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Improve artifacts workspace controls

Add artifact source previews

Improve content publishing UX