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

build/admin.asset.php

@@ -1 +1 @@
-<?php return array('dependencies' => array('react', 'react-dom', 'wp-components', 'wp-element', 'wp-i18n', 'wp-icons/build/arrow-left', 'wp-icons/build/bell', 'wp-icons/build/message', 'wp-icons/build/trash', 'wp-primitives'), 'version' => '66ba2a0809137b4a1bf3');
+<?php return array('dependencies' => array('react', 'react-dom', 'wp-components', 'wp-element', 'wp-i18n', 'wp-icons/build/arrow-left', 'wp-icons/build/bell', 'wp-icons/build/message', 'wp-icons/build/trash'), 'version' => '79dab88e37717bf64790');