fix: pick a free port for the desktop backend when 30001 is taken (#1663)

electron/preload.js

@@ -14,7 +14,16 @@ function invokeAllowed(channel, ...args) {
   return ipcRenderer.invoke(channel, ...args);
 }
 
+// The embedded backend's port, picked by the main process at startup.
+const backendPortArg = process.argv.find((arg) =>
+  arg.startsWith("--termix-backend-port="),
+);
+const backendPort = backendPortArg
+  ? Number(backendPortArg.split("=")[1])
+  : undefined;
+
 contextBridge.exposeInMainWorld("electronAPI", {
+  backendPort,
   getAppVersion: () => ipcRenderer.invoke("get-app-version"),
   // Kept so the interface size can reset a zoom an earlier build applied.
   setZoomFactor: (factor) => {