feat(pay): add wait-pay float window aligned with reference theme

- Render pending orders (max 5, >=3 min left) in a top-right float button
  with hover list and countdown, gated by pay_wait_float_s
- Fix order countdown parsing expires_at as UTC
- Drop unused .paid-modal-lists-box thumb rule

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Payment/UserOrders.php

@@ -387,8 +387,9 @@ class UserOrders {
         if ($status !== Order::STATUS_PENDING || $expires_at === '') {
             return 0;
         }
-        $expires = strtotime($expires_at);
-        return $expires ? max(0, $expires - current_time('timestamp')) : 0;
+        // expires_at 以 UTC 存储（PaymentRuntime::get_order_expiry_at），按 UTC 解析并与 UTC 时间戳比较。
+        $expires = strtotime($expires_at . ' UTC');
+        return $expires ? max(0, $expires - time()) : 0;
     }
 
     /** @return array{success:bool,message?:string,data?:array} */