渠道: 移除主页/favicon 特性, Base URL 可选+完整地址

- 回退渠道 homepage/favicon 字段与代理接口(未采用)
- Base URL 改为可选: 留空按供应商默认(openai/anthropic),
  兼容兼容型渠道必须填; 填完整地址(含 /v1)时归一化去尾
- 网关 upstreamURL 兜底去 /v1, 避免路径重复

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

scripts/mockupstream/main.go

@@ -49,12 +49,6 @@ func main() {
 		fmt.Fprint(w, `{"object":"list","data":[{"id":"gpt-4o-mini","object":"model"},{"id":"gpt-4o","object":"model"},{"id":"claude-sonnet-5","object":"model"}]}`)
 	})
 
-	// favicon（演示渠道主页图标）
-	http.HandleFunc("/favicon.ico", func(w http.ResponseWriter, r *http.Request) {
-		w.Header().Set("Content-Type", "image/svg+xml")
-		fmt.Fprint(w, `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16"><rect width="16" height="16" rx="4" fill="#34d399"/><path d="M4 4.6h8v1.4H4zM4 7.3h8v1.4H4zM4 10h5v1.4H4z" fill="#09090b"/></svg>`)
-	})
-
 	// Anthropic Messages 端点（provider=anthropic 的渠道走这里）
 	http.HandleFunc("/v1/messages", func(w http.ResponseWriter, r *http.Request) {
 		body, _ := io.ReadAll(r.Body)