Add CI workflow to build the frontend

.github/workflows/build-frontend.yml

@@ -0,0 +1,40 @@
+name: Build frontend
+
+on:
+  push:
+    branches: ["master"]
+  pull_request:
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+# Cancel superseded runs on the same ref (keep master runs intact).
+concurrency:
+  group: build-frontend-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
+jobs:
+  build-frontend:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+        with:
+          extra_nix_config: |
+            experimental-features = nix-command flakes
+
+      # Mirrors `make frontend-build` (cd frontend && purs-nix bundle),
+      # running purs-nix from the flake dev shell defined in frontend/flake.nix.
+      - name: Build frontend
+        run: nix develop ./frontend --command make frontend-build
+
+      - name: Upload bundle
+        uses: actions/upload-artifact@v4
+        with:
+          name: frontend-bundle
+          path: frontend/main.js
+          if-no-files-found: error