Show a warning banner on the homepage recommending to configure the trusted headers env

src/Services/System/TrustedHostsChecker.php

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+<?php
+/*
+ * This file is part of Part-DB (https://github.com/Part-DB/Part-DB-symfony).
+ *
+ *  Copyright (C) 2019 - 2024 Jan Böhmer (https://github.com/jbtronics)
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Affero General Public License as published
+ *  by the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Affero General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Affero General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+declare(strict_types=1);
+
+namespace App\Services\System;
+
+use Symfony\Component\DependencyInjection\Attribute\Autowire;
+
+/**
+ * Checks whether the TRUSTED_HOSTS environment variable has been configured.
+ */
+final readonly class TrustedHostsChecker
+{
+    public function __construct(
+        #[Autowire('%env(TRUSTED_HOSTS)%')]
+        private string $trustedHosts,
+    ) {
+    }
+
+    /**
+     * @return bool True if TRUSTED_HOSTS is not configured (meaning Part-DB accepts requests with any Host header), false otherwise.
+     */
+    public function isTrustedHostsUnconfigured(): bool
+    {
+        return trim($this->trustedHosts) === '';
+    }
+}