Style editor Markdown with .markdown-body and add ::: design blocks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Elternbeirat.Web/Features/Posts/PostDetail.razor

@@ -15,7 +15,7 @@ else if (_post is not null)
         <div class="post-content">
             <PostDate Value="_post.Date" />
             <h2>@_post.Title</h2>
-            @Body
+            <div class="markdown-body">@Body</div>
         </div>
     </article>
 }