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/admin/editor.html.twig

@@ -37,6 +37,7 @@
         </div>
     </header>
     <div id="message" class="editor-message" role="alert" hidden></div>
+    <div id="local-backup" class="editor-message editor-backup" role="status" hidden><span id="local-backup-text"></span><span class="editor-backup-actions"><button type="button" class="secondary" id="local-backup-restore">Wiederherstellen</button><button type="button" class="text-button" id="local-backup-discard">Verwerfen</button></span></div>
     <div id="recovery" class="recovery-bar" hidden><button id="download" class="secondary">Änderungen als JSON sichern</button><button id="reload" class="secondary">Serverstand laden</button><a href="{{ path('admin_login') }}" target="_blank" rel="noopener">Erneut anmelden ↗</a></div>
     <main id="admin-main" class="editor-layout" tabindex="-1">
         <aside class="sidebar" aria-label="Bearbeitung" data-view="elements">