Enable the no-undef eslint rule (#1848)

## Description:

Enable the `no-undef` eslint rule.

Fixes #1786

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

src/client/ClientGameRunner.ts

@@ -193,7 +193,7 @@ export class ClientGameRunner {
   private lastMousePosition: { x: number; y: number } | null = null;
 
   private lastMessageTime = 0;
-  private connectionCheckInterval: NodeJS.Timeout | null = null;
+  private connectionCheckInterval: ReturnType<typeof setTimeout> | null = null;
 
   constructor(
     private lobby: LobbyConfig,