fix(frontend): 代码块折叠加高度动画，头部在视口外时滚动跟随

收起/展开用 max-height 过渡；头部仍在视口内不改滚动，在视口外则钉住收起按钮随动画跟滚。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/globals.css

@@ -1881,6 +1881,12 @@ a.j13-mention-token:hover {
   height: 2.75rem;
   pointer-events: none;
   background: linear-gradient(to bottom, transparent, var(--panel-2));
+  transition: opacity 0.28s ease;
+}
+@media (prefers-reduced-motion: reduce) {
+  .prose-content .md-code-body {
+    transition: none !important;
+  }
 }
 .prose-content .md-code-fold {
   display: flex;