feat: set up GitHub build workflow

.github/workflows/build.yml

@@ -0,0 +1,21 @@
+name: Build
+
+on:
+  push:
+    branches:
+      - main
+
+env:
+  PUBLIC_POCKETBASE_URL: http://127.0.0.1:8090
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: npm
+      - run: npm install
+      - run: npm run build