Add per-VM autostart checkbox (single-select, persisted)

Only one VM can be marked for autostart at a time — checking a new
one unchecks the previous. Matched by host+vmid since the same vmid
can exist on multiple standalone Proxmox hosts in a cluster setup.

src/main.js

@@ -229,6 +229,15 @@ ipcMain.handle('settings:browseViewerPath', async () => {
   return chosen;
 });
 
+// Only one VM can be marked for autostart at a time — identified by
+// host+vmid since the same vmid can exist on several hosts.
+ipcMain.handle('settings:getAutoConnect', () => store.get('autoConnect', null));
+
+ipcMain.handle('settings:setAutoConnect', (_e, target) => {
+  if (target) store.set('autoConnect', target);
+  else store.delete('autoConnect');
+});
+
 // ---------- SPICE helpers ----------
 
 function buildVVFile(params, vmid, client) {