Implement self-licensing (v0.3.0) and settings sub-tabs (v0.3.1)

v0.3.0 - Self-Licensing:
- Add PluginLicenseChecker singleton for license validation
- Integrate magdev/wc-licensed-product-client library
- Add license settings: server URL, key, optional secret
- Disable frontend features without valid license (except localhost)
- Add license status display with verify button in settings

v0.3.1 - Settings UI Improvements:
- Reorganize settings page with WooCommerce-style sub-tabs
- Split settings into: Plugin License, Default Settings, Notifications
- Use PHP 8 match expression for section-specific rendering

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wc-licensed-product.php

@@ -3,7 +3,7 @@
  * Plugin Name: WooCommerce Licensed Product
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wc-licensed-product
  * Description: WooCommerce plugin to sell software products using license keys with domain-based validation.
- * Version: 0.2.2
+ * Version: 0.3.1
  * Author: Marco Graetsch
  * Author URI: https://src.bundespruefstelle.ch/magdev
  * License: GPL-2.0-or-later
@@ -28,7 +28,7 @@ if (!defined('ABSPATH')) {
 }
 
 // Plugin constants
-define('WC_LICENSED_PRODUCT_VERSION', '0.2.2');
+define('WC_LICENSED_PRODUCT_VERSION', '0.3.1');
 define('WC_LICENSED_PRODUCT_PLUGIN_FILE', __FILE__);
 define('WC_LICENSED_PRODUCT_PLUGIN_DIR', plugin_dir_path(__FILE__));
 define('WC_LICENSED_PRODUCT_PLUGIN_URL', plugin_dir_url(__FILE__));