Fix remaining f-string escaping in extraction error

app.py

@@ -757,7 +757,7 @@ def process_receipt(image):
         fields_html += "</div>"
         results['fields'] = fields
     except Exception as e:
-        fields_html = f\"<div style='color: red;'>Extraction error: {e}</div>\"
+        fields_html = f"<div style='color: red;'>Extraction error: {e}</div>"
     
     # 4. Anomaly Detection
     anomaly_html = ""