Implement product visibility management: Added configuration for product visibility in navigation and footer, along with helper functions to determine visibility and generate HTML attributes. Updated footer, header, and index files to utilize these new functions, enhancing the user interface by conditionally displaying products based on their visibility settings.

backend/includes/functions.php

@@ -3,6 +3,8 @@
  * Helper functions for HexaHost.de
  */
 
+require_once __DIR__ . '/../config/products-config.php';
+
 // Sichere Session-Konfiguration
 if (session_status() === PHP_SESSION_NONE) {
     // Session-Cookie-Sicherheit