feat: implement global autoresponder plugin

tests/mail_sender_test.php

@@ -0,0 +1,17 @@
+<?php
+declare(strict_types=1);
+
+require_once PLUGIN_ROOT . '/exec/lib/MailSender.php';
+
+test('mail sender rejects subject header injection and builds utf8 message', function (): void {
+    try {
+        MailSender::buildMessage('from@example.com', 'to@example.com', "Bad\nSubject", 'Body');
+    } catch (InvalidArgumentException $e) {
+        assert_contains('subject', strtolower($e->getMessage()));
+    }
+
+    $message = MailSender::buildMessage('from@example.com', 'to@example.com', 'Temat', "Treść\nDruga linia");
+    assert_contains('Auto-Submitted: auto-replied', $message);
+    assert_contains('Content-Type: text/plain; charset=UTF-8', $message);
+    assert_contains("Subject: =?UTF-8?B?", $message);
+});