fix: peacematch 兑换用 multipart/form-data（与抓包一致）

根据实际兑换抓包修正：
- Content-Type 从 urlencoded 改回 multipart/form-data（浏览器实际用 multipart）
- postMultipart 支持组合 token csrf\x00ctn，自动拆分并合并 cvl_csrf_token+acf_ccn 到 Cookie
- 移除不再需要的 postForm 方法和 net/url 导入
- 验证：表单字段(manualID/commodityID/num/rid/ctn)与抓包完全一致，
  storedetail 确认无 preExchange 字段（时间校准修复正确）

抓包来源：/Users/yml/Desktop/抓包/douyu/7.29/电竞手册-兑换并查询信息与积分.har

internal/client/client.go

@@ -7,7 +7,6 @@ import (
 	"io"
 	"mime/multipart"
 	"net/http"
-	"net/url"
 	"reflect"
 	"strconv"
 	"strings"
@@ -430,7 +429,7 @@ func (c *DouyuClient) Exchange(cookie, commodityID string, csrf *string) (string
 		} `json:"data"`
 	}
 	if c.Activity.UseCTN {
-		// 电竞手册：用 ctn(acf_ccn) 鉴权，urlencoded 表单
+		// 电竞手册：用 ctn(acf_ccn) 鉴权，multipart 表单（与浏览器抓包一致）
 		ctn := ""
 		if csrf != nil {
 			parts := strings.SplitN(*csrf, "\x00", 2)
@@ -446,7 +445,7 @@ func (c *DouyuClient) Exchange(cookie, commodityID string, csrf *string) (string
 			}
 			ctn = ctnVal
 		}
-		if err := c.postForm(c.Activity.ExchangeURL, cookie, csrf, "正式兑换", map[string]string{
+		if err := c.postMultipart(c.Activity.ExchangeURL, cookie, csrf, "正式兑换", map[string]string{
 			"manualID":    c.Activity.ManualID,
 			"commodityID": commodityID,
 			"num":         "1",
@@ -558,11 +557,22 @@ func (c *DouyuClient) postMultipart(url, cookie string, csrf *string, label stri
 	if err != nil {
 		return err
 	}
+	// 处理组合 token：csrf\x00ctn（电竞手册用 ctn 鉴权时）
+	// 拆分后 cvl_csrf_token 和 acf_ccn 都合并到 Cookie 头
 	currentCSRF := ""
+	ctnVal := ""
 	if csrf != nil {
-		currentCSRF = *csrf
+		parts := strings.SplitN(*csrf, "\x00", 2)
+		currentCSRF = parts[0]
+		if len(parts) > 1 {
+			ctnVal = parts[1]
+		}
 	}
-	c.setBrowserHeaders(req, utils.MergeCookie(cookie, "cvl_csrf_token", currentCSRF))
+	mergedCookie := utils.MergeCookie(cookie, "cvl_csrf_token", currentCSRF)
+	if ctnVal != "" {
+		mergedCookie = utils.MergeCookie(mergedCookie, "acf_ccn", ctnVal)
+	}
+	c.setBrowserHeaders(req, mergedCookie)
 	req.Header.Set("Content-Type", writer.FormDataContentType())
 	resp, err := c.doJSON(req, out, label)
 	if resp != nil {
@@ -570,43 +580,6 @@ func (c *DouyuClient) postMultipart(url, cookie string, csrf *string, label stri
 			if nextCSRF != *csrf {
 				logger.LogEvent("网络", "info", "响应刷新CSRF已同步: %s %s %s", label, req.Method, requestPath(req))
 			}
-			*csrf = nextCSRF
-		}
-	}
-	return err
-}
-
-// postForm 发送 application/x-www-form-urlencoded POST 请求。
-// 用于电竞手册等使用 ctn 鉴权的活动。
-// csrf 参数为组合值 "cvl_csrf_token\x00acf_ccn"，两者都会合并到 Cookie 头。
-func (c *DouyuClient) postForm(targetURL, cookie string, csrf *string, label string, fields map[string]string, out any) error {
-	vals := url.Values{}
-	for k, v := range fields {
-		vals.Set(k, v)
-	}
-	req, err := http.NewRequest("POST", targetURL, strings.NewReader(vals.Encode()))
-	if err != nil {
-		return err
-	}
-	// 拆分组合 token，合并到 Cookie
-	csrfVal := ""
-	ctnVal := ""
-	if csrf != nil {
-		parts := strings.SplitN(*csrf, "\x00", 2)
-		csrfVal = parts[0]
-		if len(parts) > 1 {
-			ctnVal = parts[1]
-		}
-	}
-	mergedCookie := utils.MergeCookie(cookie, "cvl_csrf_token", csrfVal)
-	if ctnVal != "" {
-		mergedCookie = utils.MergeCookie(mergedCookie, "acf_ccn", ctnVal)
-	}
-	c.setBrowserHeaders(req, mergedCookie)
-	req.Header.Set("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8")
-	resp, err := c.doJSON(req, out, label)
-	if resp != nil && csrf != nil {
-		if nextCSRF := csrfFromResponse(resp); nextCSRF != "" {
 			if ctnVal != "" {
 				*csrf = nextCSRF + "\x00" + ctnVal
 			} else {