added stopwatch demo app

Author: sumn2u

en/beginner-projects/simple-stopwatch.md

@@ -21,3 +21,12 @@ Create a simple stopwatch app that lets users start, stop, and reset a timer to
 Add lap functionality to record and display multiple time splits.
 
 
+## Live Demo
+<div align="center">
+<iframe src="https://codesandbox.io/embed/yyhqfy?view=preview"
+     style="width:100%; height: 500px; border:0; border-radius: 4px; overflow:hidden;"
+     title="stopwatch-app"
+     allow="accelerometer; ambient-light-sensor; camera; encrypted-media; geolocation; gyroscope; hid; microphone; midi; payment; usb; vr; xr-spatial-tracking"
+     sandbox="allow-forms allow-modals allow-popups allow-presentation allow-same-origin allow-scripts"
+   ></iframe>
+   </div>