feat: migrate shipping to form-level and integrate flags.json as single source of truth

Shipping Migration:
- Move shipping configuration from product-level to form-level
- Add form shipping tab in form settings (no_shipping, flat_rate, free_shipping)
- Update FlatRate to register at form level instead of product level
- Update checkout logic to read from form settings
- Support percentage-based flat rate calculation
- Simplify shipping method IDs (flat_rate, free_shipping)

Currency Flags Integration:
- Add formipay_get_all_currency_flags() to read from admin/assets/json/flags.json
- Remove hardcoded CURRENCY_FLAGS emoji map from VariationField.js
- Create CurrencyFlag component to render base64 flag images
- Localize currency_flags to window.formipayProductDetails
- Update shipping info display in admin order details

Benefits:
- Form-level shipping prevents multiplying shipping costs per product
- Single source of truth for currency flags (flags.json)
- Better support for future cart system
- Consistent with e-commerce standards

admin/assets/js/admin-order-details.js

@@ -64,6 +64,28 @@ jQuery(function($){
                 $('#order-total').html(res.total_formatted);
                 $('#order_status').val(res.status);
 
+                // Populate shipping info if available
+                var shippingInfo = [];
+                if(res.form_data){
+                    $.each(res.form_data, function(key, data){
+                        if(data.name === 'shipping_country' || data.name === 'shipping_method'){
+                            shippingInfo.push(data);
+                        }
+                    });
+                }
+                if(shippingInfo.length > 0){
+                    var source = $("#shipping-info-template").html();
+                    var template = Handlebars.compile(source);
+                    var context = {
+                        datas: shippingInfo
+                    };
+                    var html = template(context);
+                    $("#shipping-info-list").html(html);
+                } else {
+                    $("#shipping-info-list").addClass('d-none');
+                    $('#no-shipping-info').removeClass('d-none');
+                }
+
                 var source = $("#form-data-item-template").html();
                 var template = Handlebars.compile(source);
                 var context = {