fix(post-overview): hide block entirely when no posts + no intro text

The post_overview block kept rendering its <h3> headline + the
"all posts" footer link even when both `postsResolved` was empty
and `text` was unset, leaving a heading with nothing under it on
pages like /mediathek that include the block defensively.

Wrap the whole render in `{#if hasContent}` so empty overviews
disappear; the heading reappears as soon as posts or intro text
are present.

src/lib/components/blocks/PostOverviewBlock.svelte

@@ -20,8 +20,13 @@
   const posts = $derived((block.postsResolved ?? []) as (PostEntry & { _resolvedImageUrl?: string })[]);
   const design = $derived(block.design || "cards");
   const tagBase = "/posts/tag";
+  // Hide the entire block (headline, intro, footer link) when there are no
+  // posts and no intro text — otherwise an empty section with just a heading
+  // sits on the page.
+  const hasContent = $derived(posts.length > 0 || textHtml.length > 0);
 </script>
 
+{#if hasContent}
 <div
   class="post-overview mb-4 {layoutClasses} {className}"
   data-block-type="post_overview"
@@ -57,3 +62,4 @@
     </div>
   {/if}
 </div>
+{/if}