Make unit tests an explicit CI step; bump Node, add npm cache

- Dedicated 'Run unit tests' step (npm run test:unit) so unit tests are
  visible and guaranteed in CI rather than implied by the build script.
- Build now calls react-scripts build directly, avoiding a redundant
  second Jest run (CI=true still fails the build on lint warnings).
- Bump Node 18 (EOL) -> 20 and enable npm dependency caching.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -14,10 +14,11 @@ jobs:
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
+          cache: 'npm'
       - name: Install dependencies
         run: npm ci
-      - name: Run tests
-        run: npm test -- --runInBand
+      - name: Run unit tests
+        run: npm run test:unit
       - name: Build production bundle
-        run: npm run build
+        run: npx react-scripts build