Don't block desktop app on backend starting up (#5041)

# Description of Changes
Start bundled backend instantly on startup of app and don't wait on it
being fully up to spawn app. This is techincally wasteful curently on
self-hosted mode where everything runs remotely, but in the future we'll
probably route simple operations to the local machine regardless of
connection, and it stops unnecessary waiting in the offline mode.

frontend/src/desktop/components/SetupWizard/index.tsx

@@ -61,7 +61,7 @@ export const SetupWizard: React.FC<SetupWizardProps> = ({ onComplete }) => {
       }
 
       await connectionModeService.switchToSaaS(serverConfig.url);
-      await tauriBackendService.startBackend();
+      tauriBackendService.startBackend().catch(console.error);
       onComplete();
     } catch (err) {
       console.error('SaaS login failed:', err);