feat(settings): 后台可配置 Markdown 代码块自动折叠与阈值行数

支持开关与 0–100 行阈值（0 为无预览），经站点设置与 WS 热更新；短代码在视口内收起时不再回滚浏览位置。

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

frontend/components/markdown/MdCodeBlock.tsx

@@ -11,11 +11,13 @@ import {
 } from "react";
 import { Check, Copy, ChevronDown, ChevronUp } from "lucide-react";
 import { toast } from "@/lib/toast";
+import { useCodeFoldConfig } from "./CodeFoldProvider";
 
-const COLLAPSE_LINE_THRESHOLD = 5;
-/** 约 5 行代码高度（含行高） */
-const COLLAPSED_MAX_HEIGHT = "7.75rem";
-
+/** 折叠预览高度：0=无预览；否则上下 padding ≈1.7rem + 每行 ≈1.21em */
+function collapsedMaxHeight(foldLines: number): string {
+  if (foldLines <= 0) return "0px";
+  return `calc(1.7rem + ${foldLines} * 1.21em)`;
+}
 type MdCodeBlockProps = HTMLAttributes<HTMLPreElement> & {
   children?: ReactNode;
 };
@@ -125,10 +127,11 @@ function hasShikiTokens(node: ReactNode): boolean {
   return hasShikiTokens(props.children as ReactNode);
 }
 
-/** Markdown 代码块：语言标签、复制、>5 行默认折叠；客户端仅在未真正着色时补色 */
+/** Markdown 代码块：语言标签、复制、超阈值行数默认折叠；客户端仅在未真正着色时补色 */
 export default function MdCodeBlock({ children, className, ...rest }: MdCodeBlockProps) {
   const rootRef = useRef<HTMLDivElement>(null);
   const hostRef = useRef<HTMLDivElement>(null);
+  const { enabled: autoFold, foldLines } = useCodeFoldConfig();
 
   const restMap = rest as Record<string, unknown>;
   const dataLanguage = readProp(restMap, "data-language");
@@ -176,7 +179,7 @@ export default function MdCodeBlock({ children, className, ...rest }: MdCodeBloc
   }, [plainText, className, dataLanguage, serverHighlighted]);
 
   const effectiveLines = lineCount > 0 ? lineCount : approxLines;
-  const canCollapse = foldReady && effectiveLines > COLLAPSE_LINE_THRESHOLD;
+  const canCollapse = autoFold && foldReady && effectiveLines > foldLines;
   const collapsed = canCollapse && !expanded;
 
   const onCopy = async () => {
@@ -195,11 +198,21 @@ export default function MdCodeBlock({ children, className, ...rest }: MdCodeBloc
 
   const toggleFold = () => {
     if (expanded) {
+      const el = rootRef.current;
+      // 仅当代码块顶部已滚出可视区（长代码展开后往下读）才回顶；
+      // 短代码全程在视口内时保持浏览位置不变
+      let shouldScrollBack = false;
+      if (el) {
+        const top = el.getBoundingClientRect().top;
+        const headerPad = 88; // 与 scroll-margin-top: 5.5rem 对齐
+        shouldScrollBack = top < headerPad;
+      }
       setExpanded(false);
-      // 收起后滚到该代码块顶部（含工具栏），便于继续往下读
-      requestAnimationFrame(() => {
-        rootRef.current?.scrollIntoView({ block: "start", behavior: "smooth" });
-      });
+      if (shouldScrollBack) {
+        requestAnimationFrame(() => {
+          rootRef.current?.scrollIntoView({ block: "start", behavior: "smooth" });
+        });
+      }
     } else {
       setExpanded(true);
     }
@@ -224,7 +237,7 @@ export default function MdCodeBlock({ children, className, ...rest }: MdCodeBloc
       <div
         ref={hostRef}
         className={`md-code-body${collapsed ? " md-code-collapsed" : ""}`}
-        style={collapsed ? { maxHeight: COLLAPSED_MAX_HEIGHT } : undefined}
+        style={collapsed ? { maxHeight: collapsedMaxHeight(foldLines) } : undefined}
       >
         {clientHtml ? (
           <div