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- Settings and theme refuse a write from an outdated form.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

@@ -108,7 +108,14 @@ final class Theme
         ];
     }
 
-    public function save(array $colors, array $fonts, array $base, ?array $buttons = null): void
+    /** Kennung des gespeicherten Stands, wie SiteSettings::stamp(). */
+    public function stamp(): string
+    {
+        return hash('xxh128', (string) $this->db->fetchOne('SELECT theme FROM website WHERE id = ?', ['default']));
+    }
+
+    /** @param string|null $expected die Kennung aus stamp(), als das Formular geladen wurde; null prüft nicht */
+    public function save(array $colors, array $fonts, array $base, ?array $buttons = null, ?string $expected = null): void
     {
         $theme = [
             'colors' => $this->colors($colors),
@@ -132,10 +139,18 @@ final class Theme
                 throw new InvalidPage('Die Grundschrift verweist auf eine Schrift, die es nicht gibt.');
             }
         }
-        $this->db->executeStatement('UPDATE website SET theme = ? WHERE id = ?', [
-            json_encode($theme, JSON_THROW_ON_ERROR | JSON_UNESCAPED_UNICODE),
-            'default',
-        ]);
+        $this->db->transactional(function () use ($theme, $expected): void {
+            if (null !== $expected) {
+                $lock = $this->db->getDatabasePlatform() instanceof \Doctrine\DBAL\Platforms\SQLitePlatform ? '' : ' FOR UPDATE';
+                if (!hash_equals(hash('xxh128', (string) $this->db->fetchOne('SELECT theme FROM website WHERE id = ?'.$lock, ['default'])), $expected)) {
+                    throw new \App\Domain\StaleWrite('Das Design wurde inzwischen an anderer Stelle geändert – etwa durch eine im Editor gespeicherte Stilvorlage. Die Seite zeigt jetzt den aktuellen Stand – bitte trage deine Änderung dort noch einmal ein.');
+                }
+            }
+            $this->db->executeStatement('UPDATE website SET theme = ? WHERE id = ?', [
+                json_encode($theme, JSON_THROW_ON_ERROR | JSON_UNESCAPED_UNICODE),
+                'default',
+            ]);
+        });
         // After the write, never before: saving reads the theme again, and an early reset
         // would only refill the memo with the state that is about to be replaced.
         $this->loaded = null;