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/layouts/ChatLayout.tsx

@@ -1,20 +1,13 @@
 import Footer from "../components/Footer/footer";
 import { Outlet } from "react-router";
-import SiteNav, { type User } from "../components/navbar/SiteNav";
-
-
-const userexists: User = {
-  username: "Bruno",
-  email: "",
-  avatarUrl: "",
-};
+import SiteNav from "../components/navbar/SiteNav";
 
 import styles from "./HomeLayout.module.css";
 
 export default function ChatLayout(){
     return(
         <div className={styles.root}>
-            <SiteNav user={userexists} onLogin={() => {}} onLogout={() => {}} />
+            <SiteNav />
             <Outlet />
         </div>
     )