frontend: fix — useEffect не импортирован (eslint no-undef)

frontend/src/App.js

@@ -1,4 +1,4 @@
-import React from "react";
+import React, { useEffect } from "react";
 import { BrowserRouter as Router, Routes, Route, Navigate, useLocation } from "react-router-dom";
 import AdminDashboard from "./pages/AdminDashboard";
 import DesktopSARPage from "./pages/DesktopSARPage";