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/lib/motion.ts

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+/**
+ * 全站动画策略：系统「减少动态效果」优先；站点总开关/子开关可进一步关闭。
+ * 新增性能相关动画时：CSS 挂 j13-reduce-*，JS 走 allowMotion / scrollBehavior。
+ */
+
+export type MotionKind = "codeFold" | "smoothScroll" | "chrome";
+
+export type MotionSiteConfig = {
+  /** 界面动画总开关 */
+  ui_animations: boolean;
+  /** 代码折叠高度与跟滚 */
+  anim_code_fold: boolean;
+  /** 平滑滚动 */
+  anim_smooth_scroll: boolean;
+  /** 装饰入场：列表/Toast/点赞/脉冲等 */
+  anim_chrome: boolean;
+};
+
+export const DEFAULT_MOTION_CONFIG: MotionSiteConfig = {
+  ui_animations: true,
+  anim_code_fold: true,
+  anim_smooth_scroll: true,
+  anim_chrome: true,
+};
+
+let siteConfig: MotionSiteConfig = { ...DEFAULT_MOTION_CONFIG };
+
+export function getMotionSiteConfig(): MotionSiteConfig {
+  return siteConfig;
+}
+
+export function motionConfigFromSettings(s: {
+  ui_animations?: boolean;
+  anim_code_fold?: boolean;
+  anim_smooth_scroll?: boolean;
+  anim_chrome?: boolean;
+}): MotionSiteConfig {
+  return {
+    ui_animations: s.ui_animations !== false,
+    anim_code_fold: s.anim_code_fold !== false,
+    anim_smooth_scroll: s.anim_smooth_scroll !== false,
+    anim_chrome: s.anim_chrome !== false,
+  };
+}
+
+export function osPrefersReducedMotion(): boolean {
+  return (
+    typeof window !== "undefined" &&
+    window.matchMedia("(prefers-reduced-motion: reduce)").matches
+  );
+}
+
+/** 是否允许指定类动画（OS reduce 或总开关关闭则一律否） */
+export function allowMotion(kind: MotionKind = "chrome"): boolean {
+  if (osPrefersReducedMotion()) return false;
+  if (!siteConfig.ui_animations) return false;
+  switch (kind) {
+    case "codeFold":
+      return siteConfig.anim_code_fold;
+    case "smoothScroll":
+      return siteConfig.anim_smooth_scroll;
+    case "chrome":
+      return siteConfig.anim_chrome;
+    default:
+      return true;
+  }
+}
+
+export function scrollBehavior(): ScrollBehavior {
+  return allowMotion("smoothScroll") ? "smooth" : "auto";
+}
+
+/** 同步 documentElement class，供 CSS 与 SSR 初值一致 */
+export function syncMotionHtmlClasses(cfg: MotionSiteConfig = siteConfig): void {
+  if (typeof document === "undefined") return;
+  const root = document.documentElement;
+  root.classList.toggle("j13-reduce-motion", !cfg.ui_animations);
+  root.classList.toggle(
+    "j13-reduce-chrome",
+    cfg.ui_animations && !cfg.anim_chrome
+  );
+}
+
+/** 应用站点动画配置（SSR 初值 / WS 热更新） */
+export function applyMotionSiteConfig(cfg: MotionSiteConfig): MotionSiteConfig {
+  siteConfig = motionConfigFromSettings(cfg);
+  syncMotionHtmlClasses(siteConfig);
+  return siteConfig;
+}