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/main.tsx

@@ -1,11 +1,14 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
+import { AuthProvider } from './context/AuthContext'
 import './index.css'
 import App from './App.tsx'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <App />
+    <AuthProvider>
+      <App />
+    </AuthProvider>
   </StrictMode>,
 )