GUI: fix dev-server port-retry crash under Express 5 (#3482)

Express 5's app.listen wraps the listen callback in once() and also
invokes it on 'error', with the error as the first argument — at which
point server.address() is null, so the startup log threw a TypeError
and killed the process before the EADDRINUSE handler could try the
next port. Bail out of the callback when it receives an error and let
the 'error' listener own the retry.

Verified: with 4000 occupied the server now logs the retry and comes
up on 4001; with the port free it binds 4000 as before.

src/gui/dev-server.js

@@ -86,7 +86,11 @@ const startServer = (attempt, useAnyFreePort = false) => {
         useAnyFreePort = true; // Use any port that is free
     }
 
-    const server = app.listen(useAnyFreePort ? 0 : port, () => {
+    // Express 5 invokes the listen callback on 'error' as well, with the
+    // error as its first argument — in that case server.address() is null,
+    // so bail and let the 'error' listener below own the retry.
+    const server = app.listen(useAnyFreePort ? 0 : port, (err) => {
+        if ( err ) return;
         console.log('\n-----------------------------------------------------------\n');
         console.log('Puter is now live at: ', chalk.underline.blue(`http://localhost:${server.address().port}`));
         console.log('Backend (API) origin: ', chalk.underline.blue(apiOrigin));