organize screenshot in screens directory

Moved screenshot to screens/ subdirectory for better
project organization and scalability.

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

README.md

@@ -2,7 +2,7 @@
 
 > Sichere Dokumentenverwaltung für Ihr Unternehmen - On-Premise / Selfhosting, DSGVO-konform, einfach
 
-![SafeDocs Portal Screenshot](safedocsportal-screen.jpeg)
+![SafeDocs Portal Screenshot](screens/safedocsportal-screen.jpeg)
 
 ## Über SafeDocs Portal