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- Settings and theme refuse a write from an outdated form.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/mail.html.twig

@@ -123,7 +123,7 @@
         {% if mail.protokoll is empty %}
         <span class="muted">var/log/mail.log</span>
         {% else %}
-        <form action="{{ path('admin_mail_clear') }}" method="post">
+        <form action="{{ path('admin_mail_clear') }}" method="post" data-confirm-title="Versandprotokoll leeren?" data-confirm-text="Alle Einträge in var/log/mail.log werden entfernt." data-confirm-label="Protokoll leeren" data-confirm-variant="danger">
             <input type="hidden" name="_csrf_token" value="{{ csrf_token('system') }}">
             <button class="text-button" type="submit">Protokoll leeren</button>
         </form>