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/block.html.twig

@@ -65,7 +65,8 @@
            Betrachter den Klick ab - die Seite bleibt also in jedem Fall benutzbar. #}
         <a class="image-link image-zoom" href="{{ grossesBild }}" data-zoom data-caption="{{ props.caption|default('') }}" aria-label="{{ props.alt|default('Bild'|kant_t) }} {{ 'vergrößern'|kant_t }}">
     {%- elseif imageLinked -%}
-        <a class="image-link" href="{{ props.href }}"{% include 'blocks/link-attributes.html.twig' with {newTab: props.newTab|default(false)} only %}>
+        {# Ein Bild ohne Alternativtext - ein dekoratives Logo etwa - gäbe dem Link keinen Namen. #}
+        <a class="image-link" href="{{ props.href }}"{% if props.alt|default('') == '' %} aria-label="{{ props.caption|default('') ?: (props.href == '/' ? 'Startseite'|kant_t : 'Link'|kant_t ~ ': ' ~ props.href) }}"{% endif %}{% include 'blocks/link-attributes.html.twig' with {newTab: props.newTab|default(false)} only %}>
     {%- endif -%}<img src="{{ props.src }}" alt="{{ props.alt }}" width="{{ props.width }}" height="{{ props.height }}" loading="{{ eager or inHeader|default(false) ? 'eager' : 'lazy' }}"{{ eager ? ' fetchpriority="high"' : '' }} decoding="async"{% if props.sources|default([]) %} srcset="{% for source in props.sources %}{{ source.src }} {{ source.width }}w{% if not loop.last %}, {% endif %}{% endfor %}" sizes="{{ imageSizes }}"{% endif %}>{% if klick == 'vergroessern' or imageLinked %}</a>{% endif %}{% if props.caption|default('') %}<figcaption>{{ props.caption }}</figcaption>{% endif %}</figure>
 {% elseif type == 'iconcard' %}
     <div id="{{ block.id }}" class="block-iconcard{{ designClasses }}">
@@ -298,7 +299,7 @@
 {% elseif type == 'postlist' %}
     <div id="{{ block.id }}" class="block-postlist{{ designClasses }}">
         {% set feed = postLists[block.id]|default([]) %}
-        {% include 'blocks/post-list.html.twig' with {editing: editing|default(false), posts: feed.posts|default(feed), pages: feed.pages|default(1), page: feed.page|default(1), total: feed.total|default(null), layout: block.props.layout|default('karten'), showMeta: block.props.showMeta ?? true, showExcerpt: block.props.showExcerpt ?? true, showTerms: block.props.showTerms ?? true} only %}
+        {% include 'blocks/post-list.html.twig' with {editing: editing|default(false), posts: feed.posts|default(feed), pages: feed.pages|default(1), page: feed.page|default(1), total: feed.total|default(null), layout: block.props.layout|default('karten'), showMeta: block.props.showMeta ?? true, showExcerpt: block.props.showExcerpt ?? true, showTerms: block.props.showTerms ?? true, headingLevel: block.props.headingLevel|default(3) == 2 ? 2 : 3} only %}
     </div>
 {% elseif type == 'postnav' %}
     <div id="{{ block.id }}" class="block-postnav{{ designClasses }}">