fix: 聊天居中时滚动条预留空间，避免布局偏移

Author: safe1ine

frontend/src/index.css

@@ -198,4 +198,9 @@
 
 .animate-text-flash {
   animation: text-flash 1.5s ease-in-out infinite;
+}
+
+/* 预留滚动条空间（左右对称），避免出现/消失时布局偏移，保持内容居中 */
+.scrollbar-gutter-stable {
+  scrollbar-gutter: stable both-edges;
 }

frontend/src/pages/console/user/task/task-detail.tsx

@@ -267,7 +267,7 @@ export default function TaskDetailPage() {
           <PlanStepsBlock plan={plan} streamStatus={streamStatus} />
         </div>
       )}
-      <div ref={chatScrollRef} className="flex-1 min-h-0 overflow-y-auto min-w-0">
+      <div ref={chatScrollRef} className={cn("flex-1 min-h-0 overflow-y-auto min-w-0", !hasPanel && "scrollbar-gutter-stable")}>
         <div className={cn("min-h-full", hasPanel ? "w-full" : "mx-auto max-w-[800px]")}>
           <TaskChatPanel
             scrollContainerRef={chatScrollRef}