fix: integrate updater downgrade guard

electron/main/updater.ts

@@ -115,7 +115,9 @@ export class AppUpdater extends EventEmitter {
 
     // Set channel so electron-updater requests the correct yml filename.
     // e.g. channel "alpha" → requests alpha-mac.yml, channel "latest" → requests latest-mac.yml
+    // The channel setter also enables downgrades, which automatic updates must never allow.
     autoUpdater.channel = feed.channel;
+    autoUpdater.allowDowngrade = false;
 
     if (feed.url) {
       autoUpdater.setFeedURL({
@@ -334,6 +336,7 @@ export class AppUpdater extends EventEmitter {
    */
   setChannel(channel: 'stable' | 'beta' | 'dev'): void {
     autoUpdater.channel = channel;
+    autoUpdater.allowDowngrade = false;
   }
 
   /**