Add navigation service for centralized login redirect routing

- Simplify token validation logic in API
- Update axios interceptor to use navigation service

lightrag_webui/src/services/navigation.ts

@@ -0,0 +1,17 @@
+import { NavigateFunction } from 'react-router-dom';
+
+class NavigationService {
+  private navigate: NavigateFunction | null = null;
+
+  setNavigate(navigate: NavigateFunction) {
+    this.navigate = navigate;
+  }
+
+  navigateToLogin() {
+    if (this.navigate) {
+      this.navigate('/login');
+    }
+  }
+}
+
+export const navigationService = new NavigationService();