Security audit and bug fixes (v0.12.0)

- Complete security audit for WordPress best practices, OWASP Top 10
- Fix Calculator static method calls in API controllers
- Fix EmailNotifier method names in BookingsController
- Fix guest_id type casting in EmailNotifier

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

wp-bnb.php

@@ -3,7 +3,7 @@
  * Plugin Name: WP BnB Management
  * Plugin URI: https://src.bundespruefstelle.ch/magdev/wp-bnb
  * Description: A comprehensive Bed & Breakfast management system for WordPress. Manage buildings, rooms, bookings, and guests.
- * Version: 0.11.3
+ * Version: 0.12.0
  * Requires at least: 6.0
  * Requires PHP: 8.3
  * Author: Marco Graetsch
@@ -24,7 +24,7 @@ if ( ! defined( 'ABSPATH' ) ) {
 }
 
 // Plugin version constant - MUST match Version in header above.
-define( 'WP_BNB_VERSION', '0.11.3' );
+define( 'WP_BNB_VERSION', '0.12.0' );
 
 // Plugin path constants.
 define( 'WP_BNB_PATH', plugin_dir_path( __FILE__ ) );