Keep old pages readable, detect stale settings writes, make backups honest

- Own CSS is parsed on write only. A page whose stored CSS no longer
  passes a stricter rule answers 200 and opens in the editor; that one
  block loses its CSS. A document read several times in one request is
  validated once.
- Settings and theme refuse a write from an outdated form.
- A backup fails when a media file does not arrive completely and
  leaves no half directory behind; rotation counts intact backups only.
- The archive update takes its backup behind the maintenance page.
- A package import puts all media in place before removing the old
  ones, reports honestly when that fails and keeps a maintenance state
  that was already on.
- Restoring replaces the SQLite file in one move and points at pending
  migrations.
- Six irreversible actions ask first, in a dialog of the admin's own.
- Ctrl+S in the editor takes the field being typed in along; Delete
  no longer removes a block while a dialog is open.

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

templates/admin/base.html.twig

@@ -23,6 +23,7 @@
 {%- endfor %}
     }}</script>
     <script src="/assets/color-field.js?v={{ kant_version() }}" defer></script>
+    <script src="/assets/confirm.js?v={{ kant_version() }}" defer></script>
 {% block styles %}{% endblock %}
     {% block scripts %}{% endblock %}
     {# Eigener Block, nicht Teil von `scripts`: Bildschirme wie die Mediathek ersetzen `scripts`