Run frontend build before frontend test.

"npm run build" runs in build_frontend.py too, but errors from that are suppressed,
unfortunately. This causes very slow test failures, as Playwright times
out one by one for each test.

.github/workflows/test.yaml

@@ -81,6 +81,7 @@ jobs:
       - name: Run Playwright tests
         run: |
           cd lynxkite-app/web
+          npm run build
           npm run test
 
       - uses: actions/upload-artifact@v4