feat: Move customer registration to site-level setting

Moved 'Register as site member' from order-level to site-level setting

Frontend Changes:
1. Customer Settings - Added new General section
   - Auto-register customers as site members toggle
   - Clear description of functionality
   - Saved to backend via existing API

2. OrderForm - Removed checkbox
   - Removed registerAsMember state
   - Removed checkbox UI
   - Removed register_as_member from payload
   - Backend now uses site setting

Backend Changes:
1. CustomerSettingsProvider.php
   - Added auto_register_members setting
   - Default: false (no)
   - Stored as woonoow_auto_register_members option
   - Included in get_settings()
   - Handled in update_settings()

2. OrdersController.php
   - Removed register_as_member parameter
   - Now reads from CustomerSettingsProvider
   - Site-level setting applies to all orders
   - Consistent behavior across all order creation

Benefits:
✅ Site-level control (not per-order)
✅ Consistent customer experience
✅ Easier to manage (one setting)
✅ No UI clutter in order form
✅ Setting persists across all orders

Migration:
- Old orders with checkbox: No impact
- New orders: Use site setting
- Default: Disabled (safe default)

Result:
Admins can now control customer registration site-wide from Customer Settings instead of per-order checkbox

includes/Api/OrdersController.php

@@ -780,7 +780,10 @@ class OrdersController {
     $shipping_method = sanitize_text_field( $p['shipping_method'] ?? '' ); // e.g. flat_rate:1
     $coupons         = array_filter( array_map( 'sanitize_text_field', (array) ( $p['coupons'] ?? [] ) ) );
     $note            = isset( $p['customer_note'] ) ? wp_kses_post( (string) $p['customer_note'] ) : '';
-    $register_member = (bool) ( $p['register_as_member'] ?? false );
+    
+    // Get auto-register setting from customer settings (site-level)
+    $customer_settings = \WooNooW\Compat\CustomerSettingsProvider::get_settings();
+    $register_member = $customer_settings['auto_register_members'] ?? false;
 
     // Validation: Collect all missing required fields
     $validation_errors = [];