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/components/navbar/SiteNav.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useRef, useState } from "react";
+import { useNavigate } from "react-router-dom";
 import {
   FaUserCircle,
   FaSignOutAlt,
@@ -14,21 +15,18 @@ import {
   FaHandsHelping,
 } from "react-icons/fa";
 import {FaClapperboard, FaCubes} from "react-icons/fa6";
+import { useAuth } from "@/context/AuthContext";
 import styles from "./navbar.module.css";
 
-export interface User {
-  username: string;
-  email?: string;
-  avatarUrl?: string;
-}
-
-interface NavbarProps {
-  user: User | null;
-  onLogin: () => void;
-  onLogout: () => void;
-}
-
-export default function Navbar({ user, onLogin, onLogout }: NavbarProps) {
+export default function Navbar() {
+  const { user, isAuthenticated, logout } = useAuth();
+  const navigate = useNavigate();
+  
+  const handleLogin = () => navigate("/login");
+  const handleLogout = async () => {
+    await logout();
+    navigate("/");
+  };
   const [mobileMenu, setMobileMenu] = useState(false);
   const navRef = useRef<HTMLElement | null>(null);
 
@@ -114,8 +112,8 @@ export default function Navbar({ user, onLogin, onLogout }: NavbarProps) {
         <a className={styles.linkSimple} href="#contacts"><FaGlobe className={styles.iconSmall}/> Kontakt</a>
       
         {/* right: user area */}
-        {!user ? (
-          <a className={styles.linkSimple} onClick={onLogin} aria-label="Přihlásit">
+        {!isAuthenticated ? (
+          <a className={styles.linkSimple} onClick={handleLogin} aria-label="Přihlásit">
             <FaSignInAlt className={styles.iconSmall} />
           </a>
         ) : (
@@ -138,7 +136,7 @@ export default function Navbar({ user, onLogin, onLogout }: NavbarProps) {
               <a href="/me/settings" role="menuitem">Nastavení</a>
               <a href="/me/billing" role="menuitem">Platby</a>
 
-              <button className={styles.logoutBtn} onClick={onLogout} role="menuitem">
+              <button className={styles.logoutBtn} onClick={handleLogout} role="menuitem">
                 <FaSignOutAlt className={styles.iconSmall} /> Odhlásit se
               </button>
             </div>