Improve user auth handling and download monitoring

Adds detection of local users for authentication validation and login, prevents role changes for OIDC users, and clarifies user management UI. Enhances active downloads API to include speed and ETA from qBittorrent, and improves file path handling in download monitoring. Also updates torrent tagging and user info returned by APIs.

src/app/login/page.tsx

@@ -35,6 +35,7 @@ function LoginContent() {
     backendMode: string;
     providers: string[];
     registrationEnabled: boolean;
+    hasLocalUsers: boolean;
     oidcProviderName: string | null;
   } | null>(null);
   const [showRegisterForm, setShowRegisterForm] = useState(false);
@@ -72,6 +73,7 @@ function LoginContent() {
           backendMode: 'plex',
           providers: ['plex'],
           registrationEnabled: false,
+          hasLocalUsers: false,
           oidcProviderName: null,
         });
       }