refactor(sitecode): 统一各模块分销站点编码解析为公共包

- 新增 internal/sitecode 包：FromRequest 头优先解析、FromQueryThenHeader
  探测接口专用、ExtractHost/IsLocalHost 提取 Host 域名
- 替换 auth/order/listing/chat/notification/withdrawal/wallet/distribution
  八模块 17 处各自内联的解析逻辑，统一优先级与 TrimSpace 行为
- 修复 notification 漏 TrimSpace、探测接口会话头压过显式 query、
  GetPublicSiteInfo 把 localhost Host 当站点域名查库等不一致
- 顺手清理 auth 中无引用的 extractHost 死代码
- gofmt 修复 withdrawal/presenter、chat/file/payment 测试四个历史遗留格式问题

backend/internal/modules/chat/handler_common.go

@@ -1,25 +1,14 @@
 package chat
 
 import (
-	"strings"
-
+	"hfb_sys/backend/internal/sitecode"
 	"hfb_sys/backend/pkg/response"
 
 	"github.com/gin-gonic/gin"
 )
 
 func parseSiteCode(c *gin.Context) string {
-	for _, val := range []string{
-		c.Query("distribution_site_code"),
-		c.Query("site_code"),
-		c.GetHeader("X-Distribution-Site-Code"),
-		c.GetHeader("X-Site-Code"),
-	} {
-		if trimmed := strings.TrimSpace(val); trimmed != "" {
-			return trimmed
-		}
-	}
-	return ""
+	return sitecode.FromRequest(c)
 }
 
 func (h *Handler) list(c *gin.Context, principal Principal) {