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/layout.tsx

@@ -9,6 +9,7 @@ import TipHost from "@/components/TipHost";
 import AccountGuard from "@/components/AccountGuard";
 import ThemeSync from "@/components/ThemeSync";
 import RealtimeProvider from "@/components/RealtimeProvider";
+import CodeFoldProvider from "@/components/markdown/CodeFoldProvider";
 import type { User } from "@/lib/api";
 import { getMeCached, getPublicSettingsCached } from "@/lib/serverData";
 import { TOKEN_COOKIE, authCookieHeader } from "@/lib/cookies";
@@ -115,7 +116,12 @@ export default async function RootLayout({
           tabIndex={-1}
           className="flex-1 w-full max-w-[1440px] mx-auto px-4 sm:px-6 py-7 sm:py-9 outline-none"
         >
-          {children}
+          <CodeFoldProvider
+            initialEnabled={settings.code_block_auto_fold}
+            initialFoldLines={settings.code_block_fold_lines}
+          >
+            {children}
+          </CodeFoldProvider>
         </main>
         <Footer siteName={settings.site_name} />
         <Toaster />