feat(core): add server:watch script with automatic restart on file changes (#3920)

## Description

This PR adds a new `server:watch` script that runs MagicMirror² in
server-only mode with automatic restart and browser reload capabilities.

Particularly helpful for:
- **Developers** who need to see changes immediately without manual
restarts.
- **Users setting up their mirror** who make many changes to `config.js`
or `custom.css` and need quick feedback.

### What it does

When you run `npm run server:watch`, the watcher monitors files you
specify in `config.watchTargets`. Whenever a monitored file changes:

1. The server automatically restarts
2. Waits for the port to become available
3. Sends a reload notification to all connected browsers via Socket.io
4. Browsers automatically refresh to show the changes

This creates a seamless development experience where you can edit code,
save, and see the results within seconds.

### Implementation highlights

**Zero dependencies:** Uses only Node.js built-ins (`fs.watch`,
`child_process.spawn`, `net`, `http`) - no nodemon or external watchers
needed.

**Smart file watching:** Monitors parent directories instead of files
directly to handle atomic writes from modern editors (VSCode, etc.) that
create temporary files during save operations.

**Port management:** Waits for the old server instance to fully release
the port before starting a new one, preventing "port already in use"
errors.

### Configuration

Users explicitly define which files to monitor in their `config.js`:

```js
let config = {
  watchTargets: [
    "config/config.js",
    "css/custom.css",
    "modules/MMM-MyModule/MMM-MyModule.js",
    "modules/MMM-MyModule/node_helper.js"
  ],
  // ... rest of config
};
```

This explicit approach keeps the implementation simple (~260 lines)
while giving users full control over what triggers restarts. If
`watchTargets` is empty or undefined, the watcher starts but monitors
nothing, logging a clear warning message.

---

**Note:** This PR description has been updated to reflect the final
implementation. During the review process, we refined the approach
multiple times based on feedback.

---------

Co-authored-by: Jboucly <contact@jboucly.fr>
Co-authored-by: Kristjan ESPERANTO <35647502+KristjanESPERANTO@users.noreply.github.com>

js/server_functions.js

@@ -176,4 +176,22 @@ function getEnvVars (req, res) {
 	res.send(obj);
 }
 
-module.exports = { cors, getConfig, getHtml, getVersion, getStartup, getEnvVars, getEnvVarsAsObj, getUserAgent };
+/**
+ * Get the config file path from environment or default location
+ * @returns {string} The absolute config file path
+ */
+function getConfigFilePath () {
+	// Ensure root_path is set (for standalone contexts like watcher)
+	if (!global.root_path) {
+		global.root_path = path.resolve(`${__dirname}/../`);
+	}
+
+	// Check environment variable if global not set
+	if (!global.configuration_file && process.env.MM_CONFIG_FILE) {
+		global.configuration_file = process.env.MM_CONFIG_FILE;
+	}
+
+	return path.resolve(global.configuration_file || `${global.root_path}/config/config.js`);
+}
+
+module.exports = { cors, getConfig, getHtml, getVersion, getStartup, getEnvVars, getEnvVarsAsObj, getUserAgent, getConfigFilePath };