feat: auto-login newly registered customers after checkout

- After creating new user account, immediately log them in
- Uses wp_set_current_user() and wp_set_auth_cookie()
- Provides smoother UX - customer is logged in after placing order

includes/Api/CheckoutController.php

@@ -313,6 +313,10 @@ class CheckoutController {
                         
                         // Send new account email (WooCommerce will handle this automatically via hook)
                         do_action('woocommerce_created_customer', $new_user_id, $userdata, $password);
+                        
+                        // Auto-login the newly registered user for smoother UX
+                        wp_set_current_user($new_user_id);
+                        wp_set_auth_cookie($new_user_id, true); // true = remember me
                     }
                 }
             }