Remove selected_index from gallery for better PC UX

Removed selected_index=0 so PC users see the full 3x3 grid instead
of having one image pre-selected. Mobile users can still tap any
image to view it larger.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app.py

@@ -3331,7 +3331,6 @@ if __name__ == "__main__" and not os.environ.get("QR_TESTING_MODE"):
                     object_fit="cover",
                     allow_preview=True,
                     show_download_button=False,
-                    selected_index=0,  # Open first image by default for better mobile UX
                 )
 
                 # Event handler to load settings when user clicks an example