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/board.css

@@ -224,7 +224,7 @@
 .aksrt-forum-board__application-reason {
     margin: 10px 0;
     padding: 10px 12px;
-    border-left: 3px solid var(--color-primary, #1890ff);
+    border-left: 3px solid color-mix(in srgb, var(--color-primary) 55%, transparent);
     background: var(--surface-2, #f8fafc);
     color: var(--color-ink-3, #64748b);
     font-size: 13px;
@@ -240,7 +240,7 @@
     padding: 12px 16px;
     background: var(--surface-1, #fff);
     border-radius: 8px;
-    border-left: 3px solid var(--color-primary);
+    border-left: 3px solid color-mix(in srgb, var(--color-primary) 55%, transparent);
 }
 
 .aksrt-forum-board__sticky h3 {