Add GitHub Actions workflow for Firebase deployment

- Create automated build and deploy workflow for main branch
- Uses Firebase service account stored in GitHub secrets
- Builds Astro site and deploys to Firebase Hosting
- Includes Plausible analytics tracking code in production build

Author: cursoragent

.github/workflows/deploy.yml

@@ -0,0 +1,36 @@
+name: Build and Deploy to Firebase
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+      
+      - name: Install dependencies
+        run: npm ci
+      
+      - name: Build Astro site
+        run: npm run build
+      
+      - name: Create service account file
+        run: echo '${{ secrets.FIREBASE_SERVICE_ACCOUNT }}' > $HOME/gcloud-service-key.json
+      
+      - name: Deploy to Firebase
+        run: |
+          npm install -g firebase-tools
+          firebase deploy --only hosting --non-interactive
+        env:
+          GOOGLE_APPLICATION_CREDENTIALS: ${{ github.workspace }}/gcloud-service-key.json