Add booking system with calendar and email notifications (v0.3.0)

- Booking Custom Post Type with full management features
- Room and guest relationship tracking
- Check-in/check-out date management with validation
- Booking status workflow (pending, confirmed, checked_in, checked_out, cancelled)
- Automatic price calculation using existing Calculator
- Availability system with real-time conflict detection
- AJAX endpoint for instant availability validation
- Calendar admin page with monthly view and room/building filters
- Color-coded booking status display with legend
- Email notifications for new bookings, confirmations, and cancellations
- HTML email templates with placeholder-based system
- Auto-generated booking references (BNB-YYYY-NNNNN)

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: 0.2.0
+ * Version: 0.3.0
  * 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', '0.2.0' );
+define( 'WP_BNB_VERSION', '0.3.0' );
 
 // Plugin path constants.
 define( 'WP_BNB_PATH', plugin_dir_path( __FILE__ ) );