Migrate Chat Pack shadcn Components & Theme to the `@nebari` Registry

[jbouder]

Summary

Re-source the chat pack's shadcn/ui components and theme from the Nebari design shadcn registry (the @nebari namespace) instead of the default shadcn registry. This adds a @nebari entry to registries in frontend/components.json, re-generates the vendored primitives in frontend/src/components/ui/ from that registry, and replaces the hand-authored theme tokens in frontend/src/main.css with the registry's @nebari/theme item. The shadcn CLI workflow stays — only the source registry changes.

Motivation

• Aligns the chat pack's UI primitives and theme tokens with the shared Nebari design system
• Design-system fixes, a11y improvements, and token updates arrive via shadcn add/re-sync rather than manual editing of the vendored files
• Centralizes ownership of the large :root/.dark token block in frontend/src/main.css, which is currently maintained only in this repo
• Treats installed ui/* primitives as upstream-managed source, so future updates are pulled, not re-authored
• Keeps the existing, familiar shadcn CLI tooling — low-friction adoption, no new package or build-system change

Acceptance Criteria

• @nebari registry added to the registries field in frontend/components.json ("@nebari": "https://nebari-dev.github.io/nebari-design/r/{name}.json"), which is currently {}
• The 21 vendored primitives in frontend/src/components/ui/ (accordion, avatar, badge, button, card, checkbox, dialog, dropdown-menu, field, input, label, select, separator, skeleton, sonner, spinner, switch, table, toggle, toggle-group, tooltip) re-installed from @nebari via shadcn add @nebari/<name>
• Theme tokens sourced from the @nebari/theme registry item rather than the local :root/.dark block in frontend/src/main.css
• Chat-pack-specific tokens not provided by the registry (e.g. --color-bg-neutral-*, --color-bd-brand-*, .ot-NebariChat-markdown styles) intentionally retained or migrated, not silently dropped
• style (new-york) and baseColor (neutral) in components.json reconciled with the registry's expected values
• components.json tailwind.css path (src/index.css) reconciled with the actual token file (src/main.css)
• Feature components under frontend/src/components/ render unchanged against the regenerated primitives
• npm run check (Biome), npm run build (tsc -b && vite build), and npm run ci all pass
• No visual regressions in light and dark themes

Out of Scope

• Changes to the Nebari design registry itself — missing components or tokens are addressed upstream in nebari-design, not patched locally in the vendored copies
• Backend or API changes; this is a frontend-only migration
• Chat pack layout, conversation flow, or feature redesign — this is a like-for-like re-sourcing of components and theme
• Migrating any other Nebari repo to the registry