fix: ensure frontend directory is created before deployment

.drone.yml

@@ -17,6 +17,7 @@ steps:
 
   - name: deploy
     commands:
+      - mkdir -p /opt/short-link/frontend
       - cp backend/short-link-server /opt/short-link/short-link-server.new
       - cp -r frontend/dist/* /opt/short-link/frontend/
       - mv /opt/short-link/short-link-server.new /opt/short-link/short-link-server