Add build workflow for Next.js demo

.github/workflows/build.yml

@@ -0,0 +1,22 @@
+name: Build
+
+on:
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  build-next-demo:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+
+      - name: Install dependencies and run build
+        run: npm run build