feat: add lobby UI, build integration, and test suite (Phases 9, 11, 12)

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

.github/workflows/ci.yml

@@ -33,6 +33,26 @@ jobs:
       - name: Run tests
         run: pytest -v --tb=short tests/ || true
 
+  frontend:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Build frontend
+        working-directory: frontend
+        run: |
+          npm ci
+          npm run build
+
+      - name: Run frontend tests
+        working-directory: frontend
+        run: npx vitest run
+
   java-tests:
     runs-on: ubuntu-latest
     steps: