fix(polish): 订单购物车论坛粉边软化 + 论坛付费帖购链路

- 购物车/商品/论坛表面复用 #77 soft recipe（mix 边 + soft ring）
- 论坛 visibility=pay：订单 resource + 余额扣费闭环
- VIP 价/免费解锁 + 购买 CTA/AJAX，复用现有 Order/Credits

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

assets/css/frontend/forum/topic.css

@@ -266,7 +266,10 @@
 }
 
 .aksrt-forum-topic__vote-option.is-selected {
-    outline: 1px solid var(--color-primary);
+    outline: none;
+    border-color: color-mix(in srgb, var(--color-primary) 45%, var(--color-line, #e5e7eb));
+    background: var(--color-primary-soft, color-mix(in srgb, var(--color-primary) 12%, transparent));
+    box-shadow: 0 0 0 3px color-mix(in srgb, var(--color-primary) 18%, transparent);
 }
 
 .aksrt-forum-topic__vote-count {