feat(settings): 站点界面动画总开关与子开关

管理后台可关全站动画，并细调代码折叠、平滑滚动、装饰入场；统一 lib/motion，尊重系统减少动态效果。

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

frontend/components/markdown/MdCodeBlock.tsx

@@ -11,6 +11,7 @@ import {
 } from "react";
 import { Check, Copy, ChevronDown, ChevronUp } from "lucide-react";
 import { toast } from "@/lib/toast";
+import { allowMotion } from "@/lib/motion";
 import { useCodeFoldConfig } from "./CodeFoldProvider";
 
 const FOLD_ANIM_MS = 360;
@@ -34,10 +35,7 @@ function measureCollapsedPx(host: HTMLElement, foldLines: number): number {
 }
 
 function prefersReducedMotion(): boolean {
-  return (
-    typeof window !== "undefined" &&
-    window.matchMedia("(prefers-reduced-motion: reduce)").matches
-  );
+  return !allowMotion("codeFold");
 }
 
 type MdCodeBlockProps = HTMLAttributes<HTMLPreElement> & {