fix(ci): create empty web/dist directory before backend tests

The go:embed directive in main.go requires web/dist to exist during compilation.
Backend tests don't need the actual frontend, just an empty directory to satisfy the embed directive.

.github/workflows/test-backend.yml

@@ -44,6 +44,11 @@ jobs:
         cd backend
         go mod download
 
+    - name: Create empty frontend dist directory for embed directive
+      run: |
+        mkdir -p backend/cmd/community/web/dist
+        echo '{}' > backend/cmd/community/web/dist/.gitkeep
+
     - name: Run go fmt check
       run: |
         cd backend