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- Settings and theme refuse a write from an outdated form.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

@@ -87,7 +87,7 @@
                 <div class="sheet-part">
                     <h4>Konto löschen</h4>
                     <p>Entfernt „{{ account.username }}“. Gespeicherte Inhalte bleiben unberührt – Seiten und Fassungen hängen nicht am Konto. Das eigene Konto lässt sich nicht löschen; deshalb bleibt immer mindestens ein Administrator übrig.</p>
-                    <form action="{{ path('admin_user_delete', {id: account.id}) }}" method="post">
+                    <form action="{{ path('admin_user_delete', {id: account.id}) }}" method="post" data-confirm-title="Konto „{{ account.username }}“ löschen?" data-confirm-text="Die Person kann sich danach nicht mehr anmelden. Seiten und Fassungen bleiben." data-confirm-label="Konto löschen" data-confirm-variant="danger">
                         <input type="hidden" name="_csrf_token" value="{{ csrf_token('users') }}">
                         <button class="subtle danger-text" type="submit">Konto löschen</button>
                     </form>