Only show TOC when post has h2+ headings

Short posts without section headers don't need a table of contents.
Now the TOC only appears when there are actual h2 headings to navigate.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/pages/blog/[...id].astro

@@ -86,7 +86,7 @@ const { Content, headings } = await render(post);
         ) : null
       }
     </div>
-    {headings.length > 0 && <TableOfContents headings={headings} />}
+    {headings.filter(h => h.depth === 2).length > 0 && <TableOfContents headings={headings} />}
     <article class="animate">
       <Content />
       <div class="mt-24">