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- Settings and theme refuse a write from an outdated form.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/SitePackageTest.php

@@ -90,6 +90,16 @@ final class SitePackageTest extends TestCase
         self::assertFileDoesNotExist($this->sites['ziel']['root'].'/var/maintenance');
     }
 
+    public function testAnImportLeavesAMaintenanceStateThatWasAlreadyOn(): void
+    {
+        // Wer die Wartung vorher von Hand eingeschaltet hat, will sie nach dem Import noch haben.
+        file_put_contents($this->sites['ziel']['root'].'/var/maintenance', "an\n");
+        $result = $this->sites['ziel']['package']->import($this->sites['quelle']['package']->export());
+        self::assertTrue($result['ok'], json_encode($result['steps'], JSON_UNESCAPED_UNICODE));
+        self::assertFileExists($this->sites['ziel']['root'].'/var/maintenance');
+        self::assertSame([str_repeat('a', 32).'-480.webp'], array_values(array_diff(scandir($this->sites['ziel']['root'].'/var/media'), ['.', '..'])));
+    }
+
     public function testAccountsComeAlongOnlyWhenChosenOnBothSides(): void
     {
         // Paket ohne Konten, Häkchen gesetzt: abgelehnt, nichts geändert.