Initial commit: Grav CMS setup with HTML reference material

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

plugins/email/vendor/symfony/mailer/Header/MetadataHeader.php

@@ -0,0 +1,34 @@
+<?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\Mailer\Header;
+
+use Symfony\Component\Mime\Header\UnstructuredHeader;
+
+/**
+ * @author Kevin Bond <kevinbond@gmail.com>
+ */
+final class MetadataHeader extends UnstructuredHeader
+{
+    private $key;
+
+    public function __construct(string $key, string $value)
+    {
+        $this->key = $key;
+
+        parent::__construct('X-Metadata-'.$key, $value);
+    }
+
+    public function getKey(): string
+    {
+        return $this->key;
+    }
+}