refactor: extract pkg/charset and internal/event packages

Extract ConvertChunk/FormatHexFrame into pkg/charset (zero external deps).
Extract UIEvent/UIEventKind/UIPanelKind types into internal/event.
Update all references across main package to use qualified imports.

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

mutual.go

@@ -1,15 +1,10 @@
 package main
 
 import (
-	"bytes"
-	"fmt"
 	"github.com/trzsz/trzsz-go/trzsz"
-	"github.com/zimolab/charsetconv"
 	"go.bug.st/serial"
 	"io"
 	"os"
-	"strings"
-	"time"
 )
 
 var (
@@ -21,31 +16,3 @@ var (
 	stdinPipe   *io.PipeWriter
 	clientOut   *io.PipeWriter
 )
-
-func convertChunk(chunk []byte, srcCode, dstCode string) ([]byte, error) {
-	if len(chunk) == 0 {
-		return nil, nil
-	}
-
-	if strings.EqualFold(srcCode, dstCode) {
-		dup := make([]byte, len(chunk))
-		copy(dup, chunk)
-		return dup, nil
-	}
-
-	var buf bytes.Buffer
-	err := charsetconv.ConvertWith(bytes.NewReader(chunk), charsetconv.Charset(srcCode), &buf, charsetconv.Charset(dstCode), false)
-	if err != nil {
-		return nil, err
-	}
-
-	return buf.Bytes(), nil
-}
-
-func formatHexFrame(frame []byte, withTimestamp bool, tsFmt string) string {
-	if withTimestamp {
-		return fmt.Sprintf("%v % X %q \n", time.Now().Format(tsFmt), frame, frame)
-	}
-
-	return fmt.Sprintf("% X %q \n", frame, frame)
-}