Add screenshot showing widget in action

- Add screenshots/widget-preview.png with full desktop capture
- Update README.md to display the screenshot

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -23,6 +23,8 @@ A KDE Plasma 5 widget that displays the current virtual desktop name and provide
 
 ## Screenshots
 
+![Virtual Desktop Switcher Widget](screenshots/widget-preview.png)
+
 The widget shows a grid of desktop previews with:
 - Window outlines in real positions
 - Application icons inside windows