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/chat/BaseChat.tsx

@@ -348,7 +348,7 @@ export const BaseChat = React.forwardRef<HTMLDivElement, BaseChatProps>(
               </div>
             )}
             <StickToBottom
-              className={classNames('pt-6 px-2 sm:px-6 relative', {
+              className={classNames('modern-scrollbar pt-6 px-2 sm:px-6 relative', {
                 'h-full flex flex-col': chatStarted,
               })}
               resize="smooth"