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/MobilePostBar.tsx

@@ -3,6 +3,7 @@
 import { useEffect, useState } from "react";
 import { ArrowUp, MessageSquare } from "lucide-react";
 import LikeButton from "./LikeButton";
+import { scrollBehavior } from "@/lib/motion";
 
 /**
  * 帖子详情移动端浮动操作条（lg 以下显示）：
@@ -28,7 +29,7 @@ export default function MobilePostBar({
 
   const scrollToComments = () => {
     document.getElementById("comment-input")?.scrollIntoView({
-      behavior: window.matchMedia("(prefers-reduced-motion: reduce)").matches ? "auto" : "smooth",
+      behavior: scrollBehavior(),
       block: "center",
     });
   };
@@ -36,7 +37,7 @@ export default function MobilePostBar({
   const scrollToTop = () => {
     window.scrollTo({
       top: 0,
-      behavior: window.matchMedia("(prefers-reduced-motion: reduce)").matches ? "auto" : "smooth",
+      behavior: scrollBehavior(),
     });
   };