fix bug

frontend/src/lib/client/lucifer.ts

@@ -3,7 +3,7 @@ import type CommandInput from "$lib/models/command";
 import type UIState from "$lib/models/uistate";
 
 export default async function fetchLucifer<T>(path: string, init?: RequestInit): Promise<T> {
-  const url = import.meta.env.VITE_LUCIFER4_BACKEND_URL + "/" + path;
+  const url = (import.meta.env.VITE_LUCIFER4_BACKEND_URL||"") + "/" + path;
 
   const res = await fetch(url, init);
   if (res.status !== 200) {