ci: add npm run build step to unit-tests job

The prerender fix only prevents regression if the build is actually run in
CI. Without this gate, a future prerendered route that becomes unreachable
behind auth would fail silently until someone runs the build manually.

Fits after the test step in the existing unit-tests job — no new job needed
since node_modules is already cached for the Playwright container.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -40,6 +40,10 @@ jobs:
         run: npm test
         working-directory: frontend
 
+      - name: Build frontend
+        run: npm run build
+        working-directory: frontend
+
       - name: Upload screenshots
         if: always()
         uses: actions/upload-artifact@v4