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/app/admin/settings/SettingsAdmin.tsx

@@ -2,6 +2,7 @@
 
 import { useState } from "react";
 import {
+  Code2,
   Globe,
   Loader2,
   Save,
@@ -47,13 +48,25 @@ export default function SettingsAdmin({ initial }: { initial: PublicSettings })
   const [siteName, setSiteName] = useState(initial.site_name);
   const [siteDescription, setSiteDescription] = useState(initial.site_description);
   const [allowRegister, setAllowRegister] = useState(initial.allow_register);
+  const [codeAutoFold, setCodeAutoFold] = useState(
+    initial.code_block_auto_fold !== false
+  );
+  const [foldLines, setFoldLines] = useState(
+    Number.isFinite(initial.code_block_fold_lines) ? initial.code_block_fold_lines : 5
+  );
+  const [savedFoldLines, setSavedFoldLines] = useState(
+    Number.isFinite(initial.code_block_fold_lines) ? initial.code_block_fold_lines : 5
+  );
   const [savedName, setSavedName] = useState(initial.site_name);
   const [savedDesc, setSavedDesc] = useState(initial.site_description);
   const [saving, setSaving] = useState(false);
   const [togglingReg, setTogglingReg] = useState(false);
+  const [togglingFold, setTogglingFold] = useState(false);
+  const [savingFoldLines, setSavingFoldLines] = useState(false);
 
   const dirty =
     siteName.trim() !== savedName || siteDescription.trim() !== savedDesc;
+  const foldLinesDirty = foldLines !== savedFoldLines;
 
   const saveIdentity = async () => {
     if (saving) return;
@@ -95,6 +108,49 @@ export default function SettingsAdmin({ initial }: { initial: PublicSettings })
     }
   };
 
+
+  const toggleCodeFold = async () => {
+    if (togglingFold) return;
+    const next = !codeAutoFold;
+    setTogglingFold(true);
+    try {
+      const res = await apiUpdateSiteSettings({ code_block_auto_fold: next });
+      setCodeAutoFold(res.code_block_auto_fold);
+      toast(
+        res.code_block_auto_fold
+          ? `已开启代码块自动折叠（超过 ${res.code_block_fold_lines} 行）`
+          : "已关闭代码块自动折叠",
+        "ok"
+      );
+    } catch (e) {
+      toast(e instanceof Error ? e.message : "保存失败");
+    } finally {
+      setTogglingFold(false);
+    }
+  };
+
+  const saveFoldLines = async () => {
+    if (savingFoldLines) return;
+    const raw = Math.round(Number(foldLines));
+    const n = Number.isFinite(raw) ? Math.max(0, Math.min(100, raw)) : 5;
+    setSavingFoldLines(true);
+    try {
+      const res = await apiUpdateSiteSettings({ code_block_fold_lines: n });
+      setFoldLines(res.code_block_fold_lines);
+      setSavedFoldLines(res.code_block_fold_lines);
+      toast(
+        res.code_block_fold_lines === 0
+          ? "折叠预览已设为 0 行（无预览）"
+          : `折叠阈值已设为超过 ${res.code_block_fold_lines} 行`,
+        "ok"
+      );
+    } catch (e) {
+      toast(e instanceof Error ? e.message : "保存失败");
+    } finally {
+      setSavingFoldLines(false);
+    }
+  };
+
   return (
     <div className="max-w-3xl">
       <div className="mb-5">
@@ -174,6 +230,62 @@ export default function SettingsAdmin({ initial }: { initial: PublicSettings })
           label="开放注册"
         />
       </section>
+
+
+      <section className="j13-module p-5 sm:p-6 mb-5">
+        <div className="flex items-start justify-between gap-4 flex-wrap">
+          <div className="min-w-0 flex-1">
+            <h2 className="text-[15px] font-semibold flex items-center gap-1.5" style={{ color: "var(--ink)" }}>
+              <Code2 size={15} /> 代码块自动折叠
+            </h2>
+            <p className="meta text-[12.5px] mt-1 leading-relaxed">
+              开启后，帖子与评论中超过阈值行数的 Markdown 代码块默认折叠，读者可点「展开全部」。关闭则始终完整展示。
+            </p>
+          </div>
+          <Switch
+            checked={codeAutoFold}
+            disabled={togglingFold}
+            onChange={() => void toggleCodeFold()}
+            label="代码块自动折叠"
+          />
+        </div>
+        {codeAutoFold ? (
+          <div className="mt-4 pt-4 border-t border-[var(--line)]">
+            <label className="field-label" htmlFor="fold-lines">
+              折叠行数阈值
+            </label>
+            <p className="meta text-[12.5px] mb-2.5 leading-relaxed">
+              代码块行数超过该值时折叠；折叠后约露出同样行数。设为 0 表示无预览。范围 0–100，默认 5。
+            </p>
+            <div className="flex flex-wrap items-center gap-2.5">
+              <input
+                id="fold-lines"
+                type="number"
+                min={0}
+                max={100}
+                className="field w-24"
+                value={foldLines}
+                onChange={(e) => setFoldLines(Number(e.target.value))}
+                aria-label="折叠行数阈值"
+              />
+              <span className="meta text-[12.5px]">行</span>
+              <button
+                type="button"
+                className="btn btn-line btn-sm"
+                disabled={!foldLinesDirty || savingFoldLines}
+                onClick={() => void saveFoldLines()}
+              >
+                {savingFoldLines ? (
+                  <Loader2 size={14} className="animate-spin" />
+                ) : (
+                  <Save size={14} />
+                )}
+                保存
+              </button>
+            </div>
+          </div>
+        ) : null}
+      </section>
     </div>
   );
 }