fix(post): render post.headline/subheadline when markdown body has no H1

The article template only rendered the H1 it could pluck from the
markdown body (`contentHeadlineHtml`). Posts whose body opens with H2
(or no heading) silently lost both the schema-level `headline` and
`subheadline` — they were used in SEO/JSON-LD/breadcrumb only.

Fall back to the structured fields when no inline H1 is present.

src/routes/post/[slug]/+page.svelte

@@ -77,6 +77,15 @@
     <div class="content markdown max-w-none prose prose-zinc">
       {@html data.contentHeadlineHtml}
     </div>
+  {:else if data.post.headline || data.post.subheadline}
+    <header class="content markdown max-w-none prose prose-zinc mb-4">
+      {#if data.post.headline}
+        <h1>{data.post.headline}</h1>
+      {/if}
+      {#if data.post.subheadline}
+        <p class="text-zinc-600 lead">{data.post.subheadline}</p>
+      {/if}
+    </header>
   {/if}
   {#if data.isEvent && (data.eventDateLabel || data.eventLocationText)}
     <div class="mb-6">