fix(settings): 充值套餐选项为数组时不再触发数组转字符串警告

get_payment_recharge_settings() 原先先对 pay_balance_product 做强转、再判
is_array：当该选项以数组保存时会触发 PHP "Array to string conversion" 警告，
且强转结果随即被覆盖，属无效语句。改为按类型二选一转换，输出行为不变。

src/Settings/PaymentRuntime.php

@@ -81,10 +81,12 @@ class PaymentRuntime {
     public static function get_payment_recharge_settings(): array {
         $settings = Settings::get_all();
 
-        $packages = (string) ($settings['pay_balance_product'] ?? "30|30|入门包\n98|108|推荐包");
-        if (is_array($settings['pay_balance_product'] ?? null)) {
-            $packages = self::format_product_lines((array) $settings['pay_balance_product']);
-        }
+        // 该选项可能存字符串（旧格式）或数组（结构化保存）：先判类型再转换，
+        // 避免对数组做强转触发 "Array to string conversion" 警告。
+        $raw_packages = $settings['pay_balance_product'] ?? null;
+        $packages     = is_array($raw_packages)
+            ? self::format_product_lines($raw_packages)
+            : (string) ($raw_packages ?? "30|30|入门包\n98|108|推荐包");
         $desc = sanitize_textarea_field((string) ($settings['pay_balance_desc'] ?? ''));
 
         return [