Add GitHub Actions CI workflow

Add continuous integration workflow that:
- Runs on push to main and feature branches
- Runs on pull requests to main
- Tests on Node.js 18.x and 20.x
- Installs dependencies using npm ci
- Builds both backend and frontend
- Verifies build artifacts exist
- Uploads build artifacts for inspection

This ensures the application builds successfully across
supported Node.js versions before merging.

Co-authored-by: Ona <no-reply@ona.com>

Author: spoenemann

.github/workflows/ci.yml

@@ -0,0 +1,62 @@
+name: CI
+
+on:
+  push:
+    branches: [ main, feature/** ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    name: Build and Test
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        node-version: [18.x, 20.x]
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: 'npm'
+          cache-dependency-path: app/package-lock.json
+
+      - name: Install dependencies
+        working-directory: ./app
+        run: npm ci
+
+      - name: Build backend
+        working-directory: ./app
+        run: npm run build --workspace=backend
+
+      - name: Build frontend
+        working-directory: ./app
+        run: npm run build --workspace=frontend
+
+      - name: Verify build artifacts
+        working-directory: ./app
+        run: |
+          echo "Checking backend build artifacts..."
+          test -d backend/dist || (echo "Backend dist directory not found" && exit 1)
+          test -f backend/dist/server.js || (echo "Backend server.js not found" && exit 1)
+          test -f backend/dist/discount-rules.js || (echo "Backend discount-rules.js not found" && exit 1)
+          
+          echo "Checking frontend build artifacts..."
+          test -d frontend/dist || (echo "Frontend dist directory not found" && exit 1)
+          test -f frontend/dist/index.html || (echo "Frontend index.html not found" && exit 1)
+          
+          echo "✅ All build artifacts verified successfully"
+
+      - name: Upload build artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: build-artifacts-node-${{ matrix.node-version }}
+          path: |
+            app/backend/dist/
+            app/frontend/dist/
+          retention-days: 7