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>

config.go

@@ -1,42 +1,10 @@
 package main
 
 import (
-	"fmt"
-	"os"
-	"time"
+	appconfig "github.com/jixishi/SerialTerminalForWindowsTerminal/internal/config"
 )
 
-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
-}
+// Config is an alias for appconfig.Config to keep main-package code concise.
+type Config = appconfig.Config
 
-var config Config
-
-func openLogFile() (*os.File, error) {
-	if config.enableLog {
-		path := fmt.Sprintf(config.logFilePath, config.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
-}
+var cfg = &Config{}