feat(frontend): slice 1 — foundation for MUI->shadcn migration

Web UI rework (openspec/changes/web-ui-rework). Foundation slice:
- Add @tanstack/react-table; remove orphaned recharts, d3
- Vendor shadcn primitives: tabs table dialog input label checkbox
  switch progress separator avatar textarea dropdown-menu scroll-area
- Add Vitest + @testing-library/react + jsdom harness; npm test script
- Delete no-op theme.ts shim; remove getAppTheme references
- Smoke test proves the harness

Gate: build + lint + test green.

openspec/config.yaml

@@ -1,20 +1,16 @@
 schema: spec-driven
 
-# Project context (optional)
-# This is shown to AI when creating artifacts.
-# Add your tech stack, conventions, style guides, domain knowledge, etc.
-# Example:
-#   context: |
-#     Tech stack: TypeScript, React, Node.js
-#     We use conventional commits
-#     Domain: e-commerce platform
+context: |
+  Manage: homelab media + server-operations dashboard. FastAPI backend (backend/) + Vite/React/TypeScript frontend (frontend/).
+  Frontend stack: React 18, TypeScript, Vite, TanStack Query, Tailwind CSS v4 (CSS @theme config in src/index.css), shadcn/ui (Radix primitives), lucide-react, react-router-dom, react-oidc-context.
+  MIGRATION IN PROGRESS: MUI v9 (@mui/material, @mui/icons-material, @mui/x-data-grid, @emotion/react, @emotion/styled) -> shadcn/ui + Tailwind + lucide-react. The shell (frontend/src/App.tsx) and frontend/src/components/ObservabilityPage.tsx are already migrated and are the style targets. @mui/x-data-grid (pages/Media.tsx, pages/FileBrowser.impl.tsx) migrates to TanStack Table (@tanstack/react-table) with shadcn table styling.
+  DESIGN TOKENS: src/index.css already defines a full Tailwind v4 @theme token system (light + .dark), Inter font, primary #4f8cff, radius 0.625rem. tailwind.config.cjs is minimal. theme.ts is a no-op shim, safe to delete.
+  OBSERVABILITY MODEL: Manage is a THIN dashboard. Charts/metrics/logs live in EXTERNAL, decoupled Grafana. In-app surfaces show Alertmanager alerts + Prometheus target health + Grafana deep-links (kiosk iframe). Do NOT re-implement charting in-app. No recharts/d3 is in use.
 
-# Per-artifact rules (optional)
-# Add custom rules for specific artifacts.
-# Example:
-#   rules:
-#     proposal:
-#       - Keep proposals under 500 words
-#       - Always include a "Non-goals" section
-#     tasks:
-#       - Break tasks into chunks of max 2 hours
+rules:
+  proposal:
+    - Always include a "Non-goals" section
+    - Call out the DataGrid (TanStack Table) migration as the key technical risk
+  tasks:
+    - Slice work into chained PRs of at most 400 changed lines each
+    - Each slice must leave `npm run build` (tsc -b + vite build) and `npm run lint` green