Fix general settings route and req.user references

- Update frontend settings service to use correct /api/admin/settings/general route
- Fix all req.user to req.admin references in adminSettings.js
- Ensures settings can be saved without authentication errors

frontend/src/services/settings.service.ts

@@ -74,7 +74,7 @@ export const settingsService = {
 
   // Update multiple settings at once
   async updateSettings(settings: Record<string, any>): Promise<void> {
-    await api.put('/api/admin/settings', settings);
+    await api.put('/api/admin/settings/general', settings);
   },
 
   // Get storage information