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/CodeFoldProvider.tsx

@@ -0,0 +1,78 @@
+"use client";
+
+import {
+  createContext,
+  useContext,
+  useEffect,
+  useState,
+  type ReactNode,
+} from "react";
+import { realtime, RT_SETTINGS_CHANGED, type RtSettingsData } from "@/lib/realtime";
+
+export type CodeFoldConfig = {
+  /** 是否启用自动折叠 */
+  enabled: boolean;
+  /** 超过该行数则折叠（预览高度约同；0=无预览） */
+  foldLines: number;
+};
+
+const DEFAULT_CONFIG: CodeFoldConfig = { enabled: true, foldLines: 5 };
+
+const CodeFoldContext = createContext<CodeFoldConfig>(DEFAULT_CONFIG);
+
+/** 代码块自动折叠配置（站点设置） */
+export function useCodeFoldConfig(): CodeFoldConfig {
+  return useContext(CodeFoldContext);
+}
+
+function clampFoldLines(n: unknown): number {
+  if (typeof n !== "number" || !Number.isFinite(n)) return DEFAULT_CONFIG.foldLines;
+  const r = Math.round(n);
+  if (r < 0 || r > 100) return DEFAULT_CONFIG.foldLines;
+  return r;
+}
+
+/**
+ * 根布局注入：SSR 初值 + settings:changed 热更新。
+ */
+export default function CodeFoldProvider({
+  initialEnabled = true,
+  initialFoldLines = 5,
+  children,
+}: {
+  initialEnabled?: boolean;
+  initialFoldLines?: number;
+  children: ReactNode;
+}) {
+  const [config, setConfig] = useState<CodeFoldConfig>(() => ({
+    enabled: initialEnabled !== false,
+    foldLines: clampFoldLines(initialFoldLines),
+  }));
+
+  useEffect(() => {
+    setConfig({
+      enabled: initialEnabled !== false,
+      foldLines: clampFoldLines(initialFoldLines),
+    });
+  }, [initialEnabled, initialFoldLines]);
+
+  useEffect(() => {
+    return realtime.on<RtSettingsData>(RT_SETTINGS_CHANGED, (data) => {
+      if (!data) return;
+      setConfig((prev) => ({
+        enabled:
+          typeof data.code_block_auto_fold === "boolean"
+            ? data.code_block_auto_fold
+            : prev.enabled,
+        foldLines:
+          typeof data.code_block_fold_lines === "number"
+            ? clampFoldLines(data.code_block_fold_lines)
+            : prev.foldLines,
+      }));
+    });
+  }, []);
+
+  return (
+    <CodeFoldContext.Provider value={config}>{children}</CodeFoldContext.Provider>
+  );
+}