fix: add lineNumbers: true to rehype-pretty-code config

- Adds lineNumbers: true so data-line-numbers attribute is generated
- Enables CSS counter-based line numbers in code blocks
- Follows up on mega-review finding

app/posts/[slug]/page.tsx

@@ -92,6 +92,7 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
                           dark: 'github-dark-dimmed',
                         },
                         keepBackground: false,
+                        lineNumbers: true,
                         grid: true,
                         onVisitLine(node: LineElement) {
                           if (node.children.length === 0) {