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>

utils.go

@@ -34,13 +34,13 @@ func checkPortAvailability(name string) ([]string, error) {
 
 func OpenSerial() error {
 	mode := &serial.Mode{
-		BaudRate: config.baudRate,
-		StopBits: serial.StopBits(config.stopBits),
-		DataBits: config.dataBits,
-		Parity:   serial.Parity(config.parityBit),
+		BaudRate: cfg.BaudRate,
+		StopBits: serial.StopBits(cfg.StopBits),
+		DataBits: cfg.DataBits,
+		Parity:   serial.Parity(cfg.ParityBit),
 	}
 	var err error
-	serialPort, err = serial.Open(config.portName, mode)
+	serialPort, err = serial.Open(cfg.PortName, mode)
 	return err
 }