Back up unsaved editor work locally, structure the settings, fix archive accessibility

- The editor keeps unsaved changes in the browser and offers them
  after a crash, a closed tab or an expired session. No timed save on
  the server: every save is a full revision.
- The save status stays visible in narrow windows.
- Settings get a sticky section bar, two folded groups and a save bar
  that stays in view.
- The admin post list fetches terms and images once, not per post.
- Archive post titles are second-level headings, the archive head
  keeps full text contrast, the skip link sits above a sticky header,
  a linked image without alternative text names its link.
- The editor check starts through the dev router like the others.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

templates/blocks/post-list.html.twig

@@ -18,7 +18,9 @@
         </a>
         {% endif %}
         <div class="post-body">
-            <h3><a href="{{ post.href }}">{{ post.title }}</a></h3>
+            {# Im Archiv folgt die Liste direkt auf die Hauptüberschrift, dort ist ein Beitrag eine
+               Überschrift zweiter Ebene. In einer Seite steht sie unter einer Abschnittsüberschrift. #}
+            <h{{ headingLevel|default(3) }}><a href="{{ post.href }}">{{ post.title }}</a></h{{ headingLevel|default(3) }}>
             {% if (showMeta ?? true) and (post.publishedAt or post.author) %}
             <p class="post-meta">
                 {% if post.publishedAt %}<time datetime="{{ post.publishedAt|date('Y-m-d') }}">{{ post.publishedAt|date('d.m.Y') }}</time>{% endif %}