Fix NOVACPX_ROOT undefined on panel pages (black screen)

Constants were only defined in api/index.php, so direct requests to
admin/reseller/user index.php got a fatal error before rendering any HTML.

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

panel/public/admin/index.php

@@ -1,5 +1,7 @@
 <?php
 // NovaCPX Admin Panel — Datacenter/Server Manager
+if (!defined('NOVACPX_ROOT'))    define('NOVACPX_ROOT',    dirname(__DIR__));
+if (!defined('NOVACPX_VERSION')) define('NOVACPX_VERSION', trim(@file_get_contents(NOVACPX_ROOT . '/VERSION') ?: '1.0.0'));
 $_v = fn($f) => '?v=' . @filemtime(dirname(__DIR__) . $f);
 require_once dirname(__DIR__) . '/_branding.php';
 ?>