fix: shipping cost applied to orders + dynamic field rendering

Shipping Fix:
- Frontend now sends shipping_cost and shipping_title in order payload
- Backend uses these values as fallback when WC zone-based rate lookup fails
- Fixes issue where Rajaongkir and other API-based shipping wasn't applied

Dynamic Field Rendering:
- Added billingFields/shippingFields filters sorted by priority
- Added getBillingField/getShippingField helpers that return undefined for hidden fields
- All standard fields now conditionally rendered based on API response
- Fields use labels and required flags from API
- Any field can be hidden via PHP snippet (type: 'hidden' or hidden: true)
- Removed unused isFieldHidden function

includes/Api/CheckoutController.php

@@ -426,6 +426,23 @@ class CheckoutController {
                     'taxes'        => $rate->get_taxes(),
                 ]);
                 $order->add_item($item);
+            } elseif (!empty($payload['shipping_cost']) && $payload['shipping_cost'] > 0) {
+                // Fallback: use shipping_cost directly from frontend
+                // This handles API-based shipping like Rajaongkir where WC zones don't apply
+                $item = new \WC_Order_Item_Shipping();
+                
+                // Parse method ID from shipping_method (format: "method_id:instance_id" or "method_id:instance_id:variant")
+                $parts = explode(':', $payload['shipping_method']);
+                $method_id = $parts[0] ?? 'shipping';
+                $instance_id = isset($parts[1]) ? (int)$parts[1] : 0;
+                
+                $item->set_props([
+                    'method_title' => sanitize_text_field($payload['shipping_title'] ?? 'Shipping'),
+                    'method_id'    => sanitize_text_field($method_id),
+                    'instance_id'  => $instance_id,
+                    'total'        => floatval($payload['shipping_cost']),
+                ]);
+                $order->add_item($item);
             }
         }