feat(frontend): slice 8 — remove MUI/@emotion deps + update REQUIREMENTS

Web UI rework. Final slice.
- Remove from package.json: @mui/material, @mui/icons-material,
  @mui/x-data-grid, @emotion/react, @emotion/styled (zero consumers
  remain in src after slices 1-7b; grep-verified).
- Update docs/REQUIREMENTS.md (+63 lines): single design system
  (shadcn/ui + Tailwind v4 + lucide-react), thin-dashboard observability
  model (no in-app charts; Grafana deep-links), TanStack tables
  (visibility-only parity), reconciled IA (Backups top-level nav;
  Media at /media with /applications redirect), repurposed chart-*
  status cues, removed deps list.
- Note: AGENTS.md 'node --test tests' invocation is a pre-existing
  broken command (treats tests/ as a module); correct form is
  'node --test' (auto-discover, 5/5 pass) — verified identical at
  pre-rework baseline ef5311b.

Gate: build + lint + vitest (23/64) + node --test (5/5) green.
Web UI rework complete.

frontend/package.json

@@ -13,12 +13,7 @@
     "test:node": "node --test tests/*.test.mjs"
   },
   "dependencies": {
-    "@emotion/react": "^11.14.0",
-    "@emotion/styled": "^11.14.1",
     "@fontsource-variable/geist": "^5.2.8",
-    "@mui/icons-material": "^9.0.0",
-    "@mui/material": "^9.0.0",
-    "@mui/x-data-grid": "^9.0.4",
     "@tanstack/react-query": "^5.100.6",
     "@tanstack/react-table": "^8.21.3",
     "class-variance-authority": "^0.7.1",