Stand: SMTP-Test, Admin-Mail-Tab, Notifiable-Fix, Lazy-Quill

- Fix: Notifiable-Trait zum User-Model hinzugefuegt (behebt notify()-500er)
- Installer: SMTP-Verbindungstest mit EsmtpTransport + Ueberspringen-Link
- Admin: Neuer E-Mail-Tab mit SMTP-Konfiguration + Verbindungstest
- Admin: Lazy Quill-Initialisierung (nur sichtbare Locale wird geladen)
- Uebersetzungen: 17 neue Mail-Keys in allen 6 Sprachen

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/symfony/mime/Part/Multipart/RelatedPart.php

@@ -0,0 +1,55 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Part\Multipart;
+
+use Symfony\Component\Mime\Part\AbstractMultipartPart;
+use Symfony\Component\Mime\Part\AbstractPart;
+
+/**
+ * @author Fabien Potencier <fabien@symfony.com>
+ */
+final class RelatedPart extends AbstractMultipartPart
+{
+    public function __construct(
+        private AbstractPart $mainPart,
+        AbstractPart $part,
+        AbstractPart ...$parts,
+    ) {
+        $this->prepareParts($part, ...$parts);
+
+        parent::__construct($part, ...$parts);
+    }
+
+    public function getParts(): array
+    {
+        return array_merge([$this->mainPart], parent::getParts());
+    }
+
+    public function getMediaSubtype(): string
+    {
+        return 'related';
+    }
+
+    private function generateContentId(): string
+    {
+        return bin2hex(random_bytes(16)).'@symfony';
+    }
+
+    private function prepareParts(AbstractPart ...$parts): void
+    {
+        foreach ($parts as $part) {
+            if (!$part->getHeaders()->has('Content-ID')) {
+                $part->getHeaders()->setHeaderBody('Id', 'Content-ID', $this->generateContentId());
+            }
+        }
+    }
+}