fix: add up-to-date state so Check for updates gives visible feedback

When electron-updater reports no update available, the service now
transitions to 'up-to-date' instead of reverting silently to 'idle'.
The troubleshooting UI shows a green check icon, 'Up to date' label,
and 'You are running the latest version of Aryx.' description.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/services/autoUpdater.ts

@@ -151,7 +151,7 @@ export class AutoUpdateService {
   };
 
   private readonly notAvailableListener = () => {
-    this.publishStatus({ state: 'idle' });
+    this.publishStatus({ state: 'up-to-date' });
   };
 
   private readonly progressListener = (progress: AutoUpdateProgressLike) => {