chore: harden build and deployment workflow

.gitea/workflows/backend-tests.yml

@@ -1,4 +1,4 @@
-name: Backend tests
+name: Build and tests
 
 on:
   push:
@@ -20,10 +20,23 @@ jobs:
         with:
           node-version-file: .nvmrc
           cache: npm
-          cache-dependency-path: backend/package-lock.json
+          cache-dependency-path: |
+            backend/package-lock.json
+            frontend/package-lock.json
       - name: Install dependencies
         run: npm ci
       - name: Audit production dependencies
         run: npm audit --omit=dev
       - name: Run backend tests
         run: npm test
+      - name: Install frontend dependencies
+        working-directory: frontend
+        run: npm ci
+      - name: Audit frontend dependencies
+        working-directory: frontend
+        run: npm audit
+      - name: Build frontend
+        working-directory: frontend
+        run: npm run build
+      - name: Validate deployment script syntax
+        run: bash -n ../install.sh