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/page-order-details.php

@@ -55,6 +55,26 @@
                         </div>
                     </div>
                 </div>
+                <div class="order-detail-card shipping-info-card">
+                    <div class="card-title mt-3 mb-0"><?php echo esc_html__( 'Shipping Information', 'formipay' ); ?></div>
+                    <div class="card mt-1 border-0 rounded-4 shadow-sm">
+                        <div class="card-body p-0 placeholder-glow">
+                            <ul class="list-group list-group-flush" id="shipping-info-list">
+                                <li class="list-group-item">
+                                    <b><span class="placeholder col-3"></span></b>
+                                    <p class="mb-0"><span class="placeholder col-8"></span></p>
+                                </li>
+                                <li class="list-group-item">
+                                    <b><span class="placeholder col-3"></span></b>
+                                    <p class="mb-0"><span class="placeholder col-8"></span></p>
+                                </li>
+                            </ul>
+                            <div class="d-none" id="no-shipping-info">
+                                <p class="text-center text-muted my-3"><?php echo esc_html__( 'No shipping information available', 'formipay' ); ?></p>
+                            </div>
+                        </div>
+                    </div>
+                </div>
                 <div class="order-detail-card form-data-card">
                     <div class="card-title mt-3 w-100 mb-0 d-flex justify-content-between align-items-center">
                         <?php echo esc_html__( 'Form Data', 'formipay' ); ?> 
@@ -273,4 +293,12 @@
         <b><?php echo esc_html__('Access Password', 'formipay'); ?></b>
         <p class="mb-0">******</p>
     </li>
+</script>
+<script id="shipping-info-template" type="text/x-handlebars-template">
+    {{#each datas as |data|}}
+        <li class="list-group-item px-0">
+            <b class="field-name">{{data.label}}</b>
+            <p class="field-value mt-1 mb-0">{{data.value}}</p>
+        </li>
+    {{/each}}
 </script>