重构会话页面的消息渲染，支持块级虚拟化，提升长会话性能，减少卡断

src/components/ai-elements/reasoning.tsx

@@ -139,7 +139,7 @@ export const Reasoning = memo(
     return (
       <ReasoningContext.Provider value={contextValue}>
         <Collapsible
-          className={cn("not-prose mb-4", className)}
+          className={cn("not-prose", className)}
           onOpenChange={handleOpenChange}
           open={isOpen}
           {...props}