Fix HAS_ZXING NameError: check if variable exists before using

app.py

@@ -61,7 +61,7 @@ except Exception as e:
     print(f"✗ Barcode reader import failed: {e}")
 
 # Enable barcode detection if we have ZXing-CPP or pyzbar
-if HAS_ZXING:
+if 'HAS_ZXING' in globals() and HAS_ZXING:
     HAS_BARCODE = True
     print("✓ Barcode detection enabled via ZXing-CPP")
 elif HAS_BARCODE: