feat: add -logfile flag to write logs to a file

The -logfile flag writes log output to a file in addition to stderr,
so users can run barnard in the TUI without log spam on screen.

Usage:
  barnard -log=debug -logfile=/tmp/barnard.log -server=mumble.example.com

main.go

@@ -117,6 +117,7 @@ func main() {
 	profile := flag.Bool("profile", false, "add http server to serve profiles")
 	noiseSuppressionEnabled := flag.Bool("noise-suppression", false, "enable noise suppression for microphone input")
 	logLevel := flag.String("log", "warn", "log level: debug, info, warn, error")
+	logFile := flag.String("logfile", "", "write logs to this file (in addition to stderr)")
 
 	flag.Parse()
 
@@ -134,7 +135,17 @@ func main() {
 	default:
 		level = barnlog.LevelWarn
 	}
-	barnlog.SetLogger(barnlog.NewWriterLogger(os.Stderr, level))
+	// Default: log to stderr
+	var writer io.Writer = os.Stderr
+	if *logFile != "" {
+		f, err := os.OpenFile(*logFile, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "cannot open log file %s: %v\n", *logFile, err)
+		} else {
+			writer = io.MultiWriter(os.Stderr, f)
+		}
+	}
+	barnlog.SetLogger(barnlog.NewWriterLogger(writer, level))
 
 	if *profile == true {
 		go func() {