docs: update README screenshot

Author: Producdevity

README.md

@@ -1,6 +1,6 @@
 # GreenOvercast
 
-<img width="640" height="480" alt="screenshot" src="https://github.com/user-attachments/assets/4ec93eeb-8795-44e5-9ba8-c6065adf4660" />
+<img width="640" height="480" alt="GreenOvercast game library" src="packaging/portmaster/greenovercast/screenshot.png" />
 
 GreenOvercast is a native Xbox Cloud Gaming client for small ARM64 Linux
 handhelds. It requests a stream matching the device display and uses hardware