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- Settings and theme refuse a write from an outdated form.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>

src/Controller/SettingsController.php

@@ -57,6 +57,7 @@ final class SettingsController extends AbstractController
 
         return $this->render('admin/settings.html.twig', [
             'settings' => $settings,
+            'settingsStamp' => $this->settings->stamp(),
             'businessTypes' => SiteSettings::BUSINESS_TYPES,
             'legalSelection' => $legal,
             'notFoundSelection' => $this->notFoundPage->selected(),
@@ -124,8 +125,9 @@ final class SettingsController extends AbstractController
             return $this->back('Deine Sitzung ist abgelaufen. Bitte melde dich erneut an.');
         }
         try {
-            $this->settings->save($request->request->all());
-        } catch (InvalidPage $e) {
+            // Ohne Kennung (ein Formular von vor dem Update) wird nicht geprüft, sondern gespeichert.
+            $this->settings->save($request->request->all(), $request->request->getString('expected') ?: null);
+        } catch (InvalidPage|StaleWrite $e) {
             return $this->back($e->getMessage());
         }