feat: add clear log for occupied port at startup (#3890)

Having repeatedly seen that users are unaware of the meaning of the
EADDRINUSE error message (see, for example, this [forum
thread](https://forum.magicmirror.builders/topic/19871/update-package-list/5)),
I thought we should intercept this message and provide clearer output.
This may help users identify the cause of the problem more quickly
themselves.

## before

```
[2025-09-13 09:54:32.903] [LOG]   Starting MagicMirror: v2.33.0-develop 
...
[2025-09-13 09:54:33.533] [LOG]   Starting server on port 8080 ...  
[2025-09-13 09:54:33.537] [WARN]  You're using a full whitelist configuration to allow for all IPs 
[2025-09-13 09:54:33.568] [ERROR] Whoops! There was an uncaught exception... 
[2025-09-13 09:54:33.574] [ERROR] Error: listen EADDRINUSE: address already in use 0.0.0.0:8080
    at Server.setupListenHandle [as _listen2] (node:net:1940:16)
    at listenInCluster (node:net:1997:12)
    at node:net:2206:7
    at process.processTicksAndRejections (node:internal/process/task_queues:90:21) {
  code: 'EADDRINUSE',
  errno: -98,
  syscall: 'listen',
  address: '0.0.0.0',
  port: 8080
} 
[2025-09-13 09:54:33.574] [ERROR] MagicMirror² will not quit, but it might be a good idea to check why this happened. Maybe no internet connection? 
[2025-09-13 09:54:33.574] [ERROR] If you think this really is an issue, please open an issue on GitHub: https://github.com/MagicMirrorOrg/MagicMirror/issues 
[2025-09-13 09:54:35.235] [INFO]  
####  System Information  ####
...
```

## after

```
[2025-09-13 09:53:20.151] [LOG]   Starting MagicMirror: v2.33.0-develop 
...
[2025-09-13 09:53:20.928] [LOG]   Starting server on port 8080 ...  
[2025-09-13 09:53:20.931] [WARN]  You're using a full whitelist configuration to allow for all IPs 
[2025-09-13 09:53:20.970] [ERROR] 
────────────────────────────────────────────────────────────────
 PORT IN USE: 0.0.0.0:8080

 Another process (most likely another MagicMirror instance)
 is already using this port.

 Stop the other process (free the port) or use a different port.
──────────────────────────────────────────────────────────────── 
[2025-09-13 09:53:22.471] [INFO]  
####  System Information  ####
...
```

Author: KristjanESPERANTO

CHANGELOG.md

@@ -17,8 +17,9 @@ Thanks to: @dathbe.
 
 ### Added
 
-- Added configuration option for `User-Agent`, used by calendar & news module (#3255)
-- [linter] Added prettier plugin for nunjuck templates (#3887)
+- Add configuration option for `User-Agent`, used by calendar & news module (#3255)
+- [linter] Add prettier plugin for nunjuck templates (#3887)
+- [core] Add clear log for occupied port at startup (#3890)
 
 ### Changed
 

js/server.js

@@ -55,6 +55,29 @@ function Server (config) {
 			});
 
 			Log.log(`Starting server on port ${port} ... `);
+
+			// Add explicit error handling BEFORE calling listen so we can give user-friendly feedback
+			server.once("error", (err) => {
+				if (err && err.code === "EADDRINUSE") {
+					const bindAddr = config.address || "localhost";
+					const portInUseMessage = [
+						"",
+						"────────────────────────────────────────────────────────────────",
+						` PORT IN USE: ${bindAddr}:${port}`,
+						"",
+						" Another process (most likely another MagicMirror instance)",
+						" is already using this port.",
+						"",
+						" Stop the other process (free the port) or use a different port.",
+						"────────────────────────────────────────────────────────────────"
+					].join("\n");
+					Log.error(portInUseMessage);
+					return;
+				}
+
+				Log.error("Failed to start server:", err);
+			});
+
 			server.listen(port, config.address || "localhost");
 
 			if (config.ipWhitelist instanceof Array && config.ipWhitelist.length === 0) {