feat: 重构博客为水墨纸质风格 + 搭建后台管理系统

- 重新设计全站 UI：parchment/ink/terracotta 水墨纸质色系，宋式 serif 排版
- 新增页面：文章列表、文章详情、分类、标签、关于
- GSAP ScrollTrigger 滚动动画 + 逐字揭示效果
- 后台管理系统 /admin：文章/分类/标签 CRUD，JSON 文件存储
- 登录认证（cookie session）
- 设计系统文档 UI.md

src/app/posts/[slug]/page.tsx

@@ -0,0 +1,29 @@
+import { notFound } from "next/navigation";
+import { posts } from "@/data/posts";
+import PostContent from "@/components/PostContent";
+
+export async function generateStaticParams() {
+  return posts.map((post) => ({ slug: post.slug }));
+}
+
+export async function generateMetadata({ params }: { params: Promise<{ slug: string }> }) {
+  const { slug } = await params;
+  const post = posts.find((p) => p.slug === slug);
+  if (!post) return {};
+  return {
+    title: post.title,
+    description: post.excerpt,
+  };
+}
+
+export default async function PostPage({ params }: { params: Promise<{ slug: string }> }) {
+  const { slug } = await params;
+  const post = posts.find((p) => p.slug === slug);
+  if (!post) notFound();
+
+  const currentIndex = posts.findIndex((p) => p.slug === slug);
+  const prevPost = currentIndex > 0 ? posts[currentIndex - 1] : null;
+  const nextPost = currentIndex < posts.length - 1 ? posts[currentIndex + 1] : null;
+
+  return <PostContent post={post} prevPost={prevPost} nextPost={nextPost} />;
+}