Refactor Admin UI to use unified IAM storage and add Shutdown hook

weed/command/admin.go

@@ -368,6 +368,8 @@ func startAdminServer(ctx context.Context, options AdminOptions, enableUI bool)
 		return fmt.Errorf("admin server forced to shutdown: %w", err)
 	}
 
+	adminServer.Shutdown()
+
 	return nil
 }