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- Settings and theme refuse a write from an outdated form.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

tests/ArchiveUpdateTest.php

@@ -95,7 +95,7 @@ final class ArchiveUpdateTest extends TestCase
         self::assertFileExists($this->root.'/var/cache/prod/running-service.php', 'The update request still loads compiled services after replacing files.');
         self::assertTrue($ergebnis['migrate'] ?? false, 'Nach dem Ersetzen fehlen noch die Migrationen.');
         self::assertSame(
-            ['Fassung ermitteln', 'Sicherung', 'Archiv laden', 'Prüfsumme', 'Entpacken', 'Wartungszustand', 'Dateien ersetzen', 'Zwischenspeicher', 'Dateien sind ersetzt'],
+            ['Fassung ermitteln', 'Archiv laden', 'Prüfsumme', 'Entpacken', 'Wartungszustand', 'Sicherung', 'Dateien ersetzen', 'Zwischenspeicher', 'Dateien sind ersetzt'],
             array_column($ergebnis['steps'], 'step'),
             'Die Reihenfolge ist der Punkt: gesichert wird vor dem ersten Handgriff, geprüft vor dem Entpacken.',
         );