Release version 1.1.16 - Singleton pattern for settings page

Fixed persistent duplicate settings page rendering by implementing proper
singleton pattern for admin class and caching settings instance.

Fixes:
- Settings page still appearing twice despite v1.1.15 fix
- Multiple instantiation of WC_TPP_Admin class
- Duplicate creation of WC_TPP_Settings instances

Changes:
- Implemented singleton pattern for WC_TPP_Admin class
- Added private static $instance property with get_instance() method
- Made WC_TPP_Admin constructor private
- Added static $settings_instance property to cache settings page
- Modified add_settings_page() to check and reuse cached settings instance
- Changed instantiation from new WC_TPP_Admin() to WC_TPP_Admin::get_instance()

Technical Details:
- Ensures only one WC_TPP_Admin instance exists throughout plugin lifecycle
- Prevents duplicate filter registrations even if woocommerce_get_settings_pages called multiple times
- Settings page object created once and reused on subsequent filter calls
- Follows WordPress/WooCommerce best practices for singleton implementation

Updated Files:
- includes/class-wc-tpp-admin.php (singleton pattern implementation)
- wc-tier-and-package-prices.php (version 1.1.16)
- composer.json (version 1.1.16)
- CHANGELOG.md (v1.1.16 section)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

wc-tier-and-package-prices.php

@@ -4,7 +4,7 @@
  * Plugin Name: WooCommerce Tier and Package Prices
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wc-tier-package-prices
  * Description: Add tier pricing and package prices to WooCommerce products with configurable quantities at fixed prices
- * Version: 1.1.15
+ * Version: 1.1.16
  * Author: Marco Graetsch
  * Author URI: https://src.bundespruefstelle.ch/magdev
  * Text Domain: wc-tier-package-prices
@@ -23,7 +23,7 @@ if (!defined('ABSPATH')) {
 
 // Define plugin constants
 if (!defined('WC_TPP_VERSION')) {
-    define('WC_TPP_VERSION', '1.1.15');
+    define('WC_TPP_VERSION', '1.1.16');
 }
 if (!defined('WC_TPP_PLUGIN_DIR')) {
     define('WC_TPP_PLUGIN_DIR', plugin_dir_path(__FILE__));