refactor: simplify TOML common config handling by removing markers

- Remove COMMON_CONFIG_MARKER_START/END constants
- Simplify config snippet addition/removal logic
- Use natural append/replace approach instead of markers
- Fix unused variable warning
- Improve user experience with cleaner config output

src/components/JsonEditor.tsx

@@ -62,7 +62,7 @@ const JsonEditor: React.FC<JsonEditorProps> = ({
 
         return diagnostics;
       }),
-    [showValidation]
+    [showValidation],
   );
 
   useEffect(() => {