style: add modern-scrollbar class to improve scrollbar appearance

Introduce the modern-scrollbar class to enhance the visual consistency of scrollbars across the application. This class provides a cleaner and more modern look for scrollbars in WebKit and Firefox browsers.

app/components/workbench/FileTree.tsx

@@ -143,7 +143,7 @@ export const FileTree = memo(
     };
 
     return (
-      <div className={classNames('text-sm', className, 'overflow-y-auto')}>
+      <div className={classNames('text-sm', className, 'overflow-y-auto modern-scrollbar')}>
         {filteredFileList.map((fileOrFolder) => {
           switch (fileOrFolder.kind) {
             case 'file': {