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

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

Elternbeirat.Web/Features/Pages/ContentPage.razor

@@ -9,7 +9,7 @@ else if (_page is not null)
 {
     <PageTitle>@_page.Title</PageTitle>
     <h1>@_page.Title</h1>
-    <article>@Body</article>
+    <article class="markdown-body">@Body</article>
     <PageEmbeds Embed="_page.Embed" />
 }
 @* No page and not unavailable: the slug was unknown; NavigationManager.NotFound()