feat(user-center): add cash wallet balance separate from credits

Introduce AKSRT_Balance (user_meta aksrt_balance + balance_log) for
Zibll-like pay_balance wallet. User center assets panel shows wallet
and points; balance gateway/card-pass/refunds credit cash when enabled.

inc/classes/class-aksrt-after-sale.php

@@ -514,15 +514,25 @@ class AKSRT_After_Sale {
         $payment_method = (string) ($order['payment_method'] ?? 'balance');
         $order_number = (string) ($order['order_number'] ?? '');
 
-        // Balance refund: credit back via AKSRT_Credits
-        if ($payment_method === 'balance' && class_exists('AKSRT_Credits')) {
-            AKSRT_Credits::add(
-                $user_id,
-                (int) round($refund_amount),
-                sprintf(__('售后退款：订单 %s', 'aksrt'), $order_number),
-                ['order_number' => $order_number, 'type' => 'refund'],
-                sprintf(__('售后退款金额 ¥%s', 'aksrt'), number_format($refund_amount, 2))
-            );
+        // Balance refund: prefer cash wallet, fall back to points.
+        if ($payment_method === 'balance') {
+            if (class_exists('AKSRT_Balance') && AKSRT_Balance::enabled()) {
+                AKSRT_Balance::add(
+                    $user_id,
+                    (float) $refund_amount,
+                    'order_refund',
+                    ['type' => 'order', 'order' => $order_number],
+                    sprintf(__('售后退款：订单 %s', 'aksrt'), $order_number)
+                );
+            } elseif (class_exists('AKSRT_Credits')) {
+                AKSRT_Credits::add(
+                    $user_id,
+                    (int) round($refund_amount),
+                    'order_refund',
+                    ['order_number' => $order_number, 'type' => 'refund'],
+                    sprintf(__('售后退款金额 ¥%s', 'aksrt'), number_format($refund_amount, 2))
+                );
+            }
         }
 
         // Deduct sales_volume for refunded items