Implement user authentication and account pages

Adds AuthContext with Orval API integration, user login/logout/register flows, and account settings page. Refactors navigation and layouts to use authentication context. Introduces new account-related pages (Login, Register, Logout, AccountSettings), updates PrivateRoute to use AuthContext, and improves error handling in Downloader. Removes obsolete context example and adds comprehensive usage documentation for AuthContext.

frontend/src/routes/PrivateRoute.tsx

@@ -1,22 +1,28 @@
 import { Navigate, Outlet, useLocation } from "react-router-dom";
-
-function getCookie(name: string): string | null {
-  const nameEQ = name + "=";
-  const ca = document.cookie.split(";").map((c) => c.trim());
-  for (const c of ca) {
-    if (c.indexOf(nameEQ) === 0) return decodeURIComponent(c.substring(nameEQ.length));
-  }
-  return null;
-}
-
-const ACCESS_COOKIE = "access_token";
+import { useAuth } from "@/context/AuthContext";
+import { FaSpinner } from "react-icons/fa";
 
 export default function PrivateRoute() {
   const location = useLocation();
-  const isLoggedIn = !!getCookie(ACCESS_COOKIE);
+  const { isAuthenticated, isLoading } = useAuth();
 
-  if (!isLoggedIn) {
+  // Zobraz loading během načítání uživatele
+  if (isLoading) {
+    return (
+      <div className="min-h-screen flex items-center justify-center bg-gray-50">
+        <div className="text-center">
+          <FaSpinner className="text-blue-500 text-5xl mx-auto mb-4 animate-spin" />
+          <p className="text-gray-600">Načítání...</p>
+        </div>
+      </div>
+    );
+  }
+
+  // Pokud není přihlášen, redirect na login (ulož původní cestu)
+  if (!isAuthenticated) {
     return <Navigate to="/login" replace state={{ from: location }} />;
   }
+
+  // Uživatel je přihlášen, renderuj chráněný obsah
   return <Outlet />;
 }