Add Next.js CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,45 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Build and export
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+
+      - name: Install Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: 24
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Lint
+        run: npm run lint
+
+      - name: Build static export
+        run: npm run build
+        env:
+          NEXT_PUBLIC_CALENDAR_KEY: ${{ secrets.NEXT_PUBLIC_CALENDAR_KEY }}
+
+      - name: Verify static export
+        run: |
+          test -d out
+          test -f out/index.html
+          test -f out/404.html