Fix TOC showing wrong headings across navigation

Problem: Table of Contents displayed headings from previously viewed
articles when navigating between posts via client-side routing.

Root cause: PostToc component's useEffect with empty dependency array
only ran once on mount, so it retained stale heading data when React
reused the component instance during navigation.

Solution: Add contentKey prop flow:
- Blog/page routes pass slug to PostLayout
- PostLayout passes contentKey as key prop to PostToc instances
- React remounts PostToc when key changes, rebuilding TOC correctly

Files changed:
- components/post-layout.tsx: Add contentKey prop and key forwarding
- app/blog/[slug]/page.tsx: Pass slug as contentKey
- app/pages/[slug]/page.tsx: Pass slug as contentKey

app/blog/[slug]/page.tsx

@@ -79,7 +79,7 @@ export default async function BlogPostPage({ params }: Props) {
   return (
     <>
       <ReadingProgress />
-      <PostLayout hasToc={hasToc}>
+      <PostLayout hasToc={hasToc} contentKey={slug}>
         <div className="space-y-8">
           {/* Main content area for Pagefind indexing */}
           <div data-pagefind-body>