style: 展开/收起按钮去掉背景胶囊样式，仅保留文字+chevron

src/components/messageStream/CollapsibleText.tsx

@@ -110,7 +110,7 @@ export function CollapsibleText({
           type="button"
           onClick={() => setExpanded((v) => !v)}
           aria-expanded={expanded}
-          className="group mt-1.5 -ml-2 inline-flex h-8 items-center gap-1 rounded-full px-2 text-[13px] font-medium text-ark-gold transition-colors duration-200 hover:bg-white/5 hover:text-ark-gold2"
+          className="group mt-1.5 inline-flex h-8 items-center gap-1 text-[13px] font-medium text-ark-gold transition-colors duration-200 hover:text-ark-gold2"
         >
           <span>{expanded ? t("showLess") : t("showMore")}</span>
           <m.span