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- Settings and theme refuse a write from an outdated form.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

@@ -82,6 +82,7 @@
 {% set emptyFont = {key: '', name: '', family: '', source: 'google', fallback: 'sans-serif', weights: [400, 700]} %}
 <form action="{{ path('admin_theme_save') }}" method="post" class="theme-form stack" data-theme-editor data-palettes="{{ palettes|json_encode|e('html_attr') }}" data-color-names="{{ colorNames|json_encode|e('html_attr') }}">
     <input type="hidden" name="_csrf_token" value="{{ csrf_token('theme') }}">
+    <input type="hidden" name="expected" value="{{ themeStamp }}">
 
     <section class="card">
         <div class="card-head">