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- Settings and theme refuse a write from an outdated form.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

@@ -196,6 +196,30 @@ final class BackupTest extends TestCase
         self::assertSame([$names[1], $names[0]], $removed);
     }
 
+    public function testADamagedBackupNeitherCountsAsIntactNorPushesOutACompleteOne(): void
+    {
+        $alt = $this->backup->create('Test', new \DateTimeImmutable('2026-09-10 10:00:00', new \DateTimeZone('UTC')));
+        $neu = $this->backup->create('Test', new \DateTimeImmutable('2026-09-11 10:00:00', new \DateTimeZone('UTC')));
+        // Fehlt eine der gezählten Mediendateien, ist die Sicherung nicht mehr unversehrt.
+        unlink($neu['path'].'/medien/bild-480.webp');
+        // Ein abgebrochener Lauf von früher: ein Ordner ohne Verzeichnis, der neueste von allen.
+        $halb = $this->backup->directory().'/sicherung-20260912-100000';
+        mkdir($halb);
+        touch($halb, time() - 7200);
+        // Und einer von eben - das kann eine Sicherung sein, die gerade geschrieben wird.
+        $laufend = $this->backup->directory().'/sicherung-20260913-100000';
+        mkdir($laufend);
+        touch($neu['path'], time() - 7200);
+
+        $zustand = array_column($this->backup->all(), 'intact', 'name');
+        self::assertSame([basename($laufend) => false, basename($halb) => false, $neu['name'] => false, $alt['name'] => true], $zustand);
+
+        // Vorher zählten die beschädigten als die neuesten, und die vollständige flog raus.
+        $entfernt = $this->backup->rotate(1);
+        self::assertEqualsCanonicalizing([basename($halb), $neu['name']], $entfernt);
+        self::assertSame([basename($laufend), $alt['name']], array_column($this->backup->all(), 'name'));
+    }
+
     public function testTwoBackupsInTheSameSecondGetTwoDirectories(): void
     {
         // This is not academic: a restore takes a safety copy, and that lands in the same second