[feat] Add rule no-undef in config file validation (#3786)

This should solve the problem #3785.

CHANGELOG.md

@@ -32,10 +32,8 @@ planned for 2025-07-01
 ### Fixed
 
 - [fix] Handle spellcheck issues (#3783)
-
-### Fixed
-
 - [calendar] fix fullday event rrule until with timezone offset (#3781)
+- [feat] Add rule `no-undef` in config file validation to fix #3785 (#3786)
 
 ### Updated
 

js/check_config.js

@@ -54,7 +54,8 @@ function checkConfigFile () {
 				globals: {
 					...globals.node
 				}
-			}
+			},
+			rules: { "no-undef": "error" }
 		},
 		configFileName
 	);