fix: use app.exit(0) instead of app.quit() when duplicate instance detected

When requestSingleInstanceLock returns false, app.quit() only schedules
an asynchronous shutdown but does not stop the current Effect pipeline.
This allows the startup sequence in DesktopApp to continue with
whenReady, backend bootstrap, and window creation — potentially causing
port conflicts and duplicate backends.

app.exit(0) immediately terminates the process, which is the correct
behavior for a duplicate instance with no windows or state to clean up.

Author: cursoragent

apps/desktop/src/app/DesktopCloudAuth.ts

@@ -292,7 +292,7 @@ const make = Effect.gen(function* () {
 
       const hasInstanceLock = yield* electronApp.requestSingleInstanceLock;
       if (!hasInstanceLock) {
-        return yield* electronApp.quit;
+        return yield* electronApp.exit(0);
       }
 
       yield* electronApp.on<[Electron.Event, string]>("open-url", (event, rawUrl) => {