feat(config): Enhance S2A configuration switch logging with detailed output

- Update success message to specifically indicate S2A configuration switch
- Add detailed logging output showing switched API endpoint
- Display concurrency, priority, and group IDs in configuration log
- Add conditional proxy information logging when proxy is enabled
- Improve user visibility into active configuration after switching profiles

backend/cmd/main.go

@@ -328,7 +328,13 @@ func handleConfig(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
-		logger.Success("配置已更新并保存到数据库", "", "config")
+		// 输出切换后的详细配置
+		logger.Success("S2A 配置已切换", "", "config")
+		logger.Info(fmt.Sprintf("  API: %s", config.Global.S2AApiBase), "", "config")
+		logger.Info(fmt.Sprintf("  并发: %d | 优先级: %d | 分组: %v", config.Global.Concurrency, config.Global.Priority, config.Global.GroupIDs), "", "config")
+		if config.Global.ProxyEnabled {
+			logger.Info(fmt.Sprintf("  代理: %s", config.Global.DefaultProxy), "", "config")
+		}
 		api.Success(w, map[string]string{"message": "配置已更新"})
 
 	default: