upd: restructrued to avoid useless concurrency and simplify the code

backend/src/engine.cpp

@@ -1 +1,14 @@
-#include "engine.h"
+#include "engine.h"
+
+#include <stack>
+#include <string>
+
+#include "bs-utility.h"
+std::vector<std::string> BookStoreEngineClass::Execute(
+    const std::string &cmd, std::stack<std::string> &login_stack) {
+  if (cmd == "quit" || cmd == "exit") {
+    BookStore_ZYM::shut_down = true;
+    return std::vector<std::string>();
+  }
+  return std::vector<std::string>({cmd});
+}