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>

assets/css/main.css

@@ -5756,11 +5756,6 @@ body:has(.modal.in) .float-btn,
     background: var(--main-border-color);
 }
 
-.paid-modal-lists-box .order-item .order-thumb {
-    width: 50px;
-    height: 50px;
-}
-
 /*----购买vip会员----*/
 .payvip-modal .user-box > li + li {
     width: calc(100% - 70px);