refactor: extract internal/config and eliminate global config var

Move Config struct to internal/config with exported fields. Replace
global var config with package-level cfg pointer. Add OpenLogFile to
config package. Add type alias Config = appconfig.Config in main
package for backward compatibility.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/config/config.go

@@ -0,0 +1,42 @@
+// Package config holds the application configuration.
+package config
+
+import (
+	"fmt"
+	"os"
+	"time"
+)
+
+// Config holds all application settings.
+type Config struct {
+	PortName    string
+	BaudRate    int
+	DataBits    int
+	StopBits    int
+	ParityBit   int
+	OutputCode  string
+	InputCode   string
+	EndStr      string
+	EnableLog   bool
+	LogFilePath string
+	ForWard     []int
+	FrameSize   int
+	TimesTamp   bool
+	TimesFmt    string
+	Address     []string
+	EnableGUI   bool
+	HotkeyMod   string
+}
+
+// OpenLogFile opens the configured log file for writing, or returns nil if logging is disabled.
+func OpenLogFile(cfg *Config) (*os.File, error) {
+	if cfg.EnableLog {
+		path := fmt.Sprintf(cfg.LogFilePath, cfg.PortName, time.Now().Format("2006_01_02T150405"))
+		f, err := os.OpenFile(path, os.O_RDWR|os.O_APPEND|os.O_CREATE, 0666)
+		if err != nil {
+			return nil, err
+		}
+		return f, nil
+	}
+	return nil, nil
+}