fix: add currency_code column to Bank Transfer transaction table

- Add currency_code column to formipay_bank_transfer_trx table schema
- Store currency code from product settings when creating transaction
- Prevents data loss by tracking which currency was used for bank transfer payments
- Matches PayPal implementation pattern for consistency

includes/Payment/BankTransfer.php

@@ -42,6 +42,7 @@ class BankTransfer extends Payment {
             `created_date` datetime DEFAULT CURRENT_TIMESTAMP,
             `form_id` int DEFAULT 0,
             `order_id` int DEFAULT 0,
+            `currency_code` text,
             `channel` text NOT NULL,
             `total` float(10, 2) DEFAULT 0,
             `unique_code` int DEFAULT 0,
@@ -63,10 +64,15 @@ class BankTransfer extends Payment {
         $table = $wpdb->prefix . 'formipay_bank_transfer_trx';
 
         if ($order_data['payment_gateway'] == 'bank_transfer') {
+
+            $currency_data = explode(':::', get_post_meta($order_data['form_id'], 'product_currency', true));
+            $currency_code = !empty($currency_data[0]) ? $currency_data[0] : formipay_default_currency('code');
+
             $submit_args = [
                 'created_date' => formipay_date('Y-m-d H:i:s', strtotime($order_data['created_date'])),
                 'form_id'      => intval($order_data['form_id']),
                 'order_id'     => intval($order_data['id']),
+                'currency_code' => sanitize_text_field($currency_code),
                 'channel'      => sanitize_text_field(str_replace($order_data['payment_gateway'].':::', '', $order_data['form_data']['payment'])),
                 'total'        => floatval($order_data['total']),
                 'unique_code'  => sanitize_text_field($this->check_unique_code()),