feat(web): frontend CI, vitest suite, navbar→React SPA shell — Phase 5

Frontend CI (closes the no-coverage gap for the TSX):
- ci.yml: new 'frontend' job — npm ci, tsc --noEmit, test:frontend, build.
- package.json: engines.node>=20, test:frontend/typecheck scripts.

vitest unit + component tests (jsdom, @testing-library/react):
- utils/charts.test.ts — tier math + every chart builder.
- utils/format.test.ts — parseAppDate, formatNumber, truncateKey, emoji helpers,
  formatRelativeTime.
- components/Navbar.test.tsx — feature-gated nav, custom pages, OIDC/maintenance
  auth gating.
- components/Announcements.test.tsx — banner rendering, ordering, dismiss +
  sessionStorage (covers behaviour moved out of the Python suite).

Navbar → React (full SPA shell):
- New Navbar/ThemeToggle/Announcements components + useNavItems hook (shared
  feature-gated nav for desktop + mobile); nav uses react-router NavLink
  (client-side nav + auto active) — drops the imperative data-nav-link bridge.
- main.tsx single root; App.tsx renders Navbar + Announcements above routed <main>.
- spa.html slimmed to SEO/config/footer shell (Jinja2 navbar/banners/theme script
  removed; #app is a plain div React fills).
- Backend: _build_config_json exposes system_announcement/network_announcement.

Tests (server-rendered nav/banner assertions → config):
- conftest.py: get_app_config() helper (robust __APP_CONFIG__ extraction).
- test_features/app/home/pages/dashboard rewritten to assert __APP_CONFIG__;
  dashboard client-rendered-stats tests replaced with a shell assertion.

Docs: REACT_MIGRATION.md Phase 5 (incl. deliberately-skipped react-query/
Storybook/Playwright), AGENTS.md Frontend section.

Verified: tsc clean, npm run build, vitest (49 passed), pytest tests/test_web
(251 passed), full suite (1459 passed), pre-commit (passed).

.github/workflows/ci.yml

@@ -36,6 +36,31 @@ jobs:
       - name: Run pre-commit
         uses: pre-commit/action@v3.0.1
 
+  frontend:
+    name: Frontend
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - uses: actions/checkout@v7
+
+      - name: Set up Node
+        uses: actions/setup-node@v5
+        with:
+          node-version: 24
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Typecheck
+        run: npx tsc --noEmit
+
+      - name: Unit tests
+        run: npm run test:frontend
+
+      - name: Build
+        run: npm run build
+
   test:
     name: Test
     runs-on: ubuntu-latest