refactor(site-url): 剩余 3 处 NEXT_PUBLIC_SITE_URL 硬编码 fallback 改用 lib/site-url

收尾 acbe3a7 的迁移：layout/docs-slug/u-username 还在各自用 ?? 'https://involutionhell.com'，统一走 lib/site-url 的 SITE_URL 常量，符合 docs/architecture/frontend-backend-separation.md:96-103 的 '生产禁止硬编码 fallback' 政策。

Author: longsizhuo

app/docs/[...slug]/page.tsx

@@ -1,4 +1,5 @@
 import { source } from "@/lib/source";
+import { SITE_URL } from "@/lib/site-url";
 import { DocsPage, DocsBody } from "fumadocs-ui/page";
 import { notFound } from "next/navigation";
 import type { Metadata } from "next";
@@ -94,8 +95,7 @@ export default async function DocPage({ params }: Param) {
   const Mdx = page.data.body;
 
   // SEO 结构化数据
-  const siteUrl =
-    process.env.NEXT_PUBLIC_SITE_URL || "https://involutionhell.com";
+  const siteUrl = SITE_URL;
   const slugPath = (slug ?? []).join("/");
   const docUrl = slugPath ? `${siteUrl}/docs/${slugPath}` : `${siteUrl}/docs`;
 

app/layout.tsx

@@ -26,8 +26,7 @@ const geistMono = localFont({
   weight: "100 900",
 });
 
-const SITE_URL =
-  process.env.NEXT_PUBLIC_SITE_URL || "https://involutionhell.com";
+import { SITE_URL } from "@/lib/site-url";
 const en_description =
   "内卷地狱（Involution Hell）是一个由开发者发起的开源学习社区，专注算法、系统设计、工程实践与技术分享，帮助华人程序员高效成长，专注真实进步。Involution Hell is an open-source community empowering builders with real-world engineering.";
 

app/u/[username]/page.tsx

@@ -17,6 +17,7 @@ import { GithubRepos, GithubReposSkeleton } from "./GithubRepos";
 import { Suspense } from "react";
 import { getTranslations } from "next-intl/server";
 import { sanitizeExternalUrl } from "@/lib/url-safety";
+import { SITE_URL } from "@/lib/site-url";
 
 interface UserView {
   id: number;
@@ -342,8 +343,7 @@ export default async function UserProfilePage({ params }: Param) {
   });
 
   // Person JSON-LD：让搜索引擎识别这是一个"个人档案"而不是普通页面，有机会走 knowledge panel
-  const siteUrl =
-    process.env.NEXT_PUBLIC_SITE_URL || "https://involutionhell.com";
+  const siteUrl = SITE_URL;
   const personJsonLd = {
     "@context": "https://schema.org",
     "@type": "Person",