fix: compute reading time from post content not title

app/posts/[slug]/page.tsx

@@ -6,7 +6,7 @@ import remarkGfm from 'remark-gfm'
 import rehypePrettyCode from 'rehype-pretty-code'
 import rehypeKatex from 'rehype-katex'
 import rehypeSlug from 'rehype-slug'
-import { getPosts, getPost, getReadingTime } from '@/lib/posts'
+import { getPosts, getPost } from '@/lib/posts'
 import { TableOfContents } from '@/components/blog/TableOfContents'
 import { ScrollToTop } from '@/components/ui/ScrollToTop'
 import { ReadingProgress } from '@/components/ui/ReadingProgress'
@@ -45,7 +45,7 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
                 {post.title}
               </h1>
               <p className="font-mono text-xs text-ink-soft">
-                {getReadingTime(post.title)} min read
+                {post.readingTime} min read
               </p>
             </header>
             <div className="prose prose-lg max-w-none">