Refactor helper functions to separate Helper class and translate comments to Korean

- Extract hex conversion functions to new Helper.cs file
- Remove duplicate helper functions from Program.cs and V2GDecoder.cs
- Update all references to use Helper class
- Translate all comments in Program.cs to Korean
- Improve code organization and reusability

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Helper.cs

@@ -0,0 +1,38 @@
+using System;
+
+namespace V2GProtocol
+{
+    /// <summary>
+    /// 헥스 문자열 변환 헬퍼 클래스
+    /// </summary>
+    public static class Helper
+    {
+        /// <summary>
+        /// 헥스 문자열을 바이트 배열로 변환
+        /// </summary>
+        /// <param name="hexString">헥스 문자열</param>
+        /// <returns>바이트 배열</returns>
+        public static byte[] FromHexString(string hexString)
+        {
+            if (hexString.Length % 2 != 0)
+                throw new ArgumentException("Invalid hex string length");
+                
+            byte[] bytes = new byte[hexString.Length / 2];
+            for (int i = 0; i < bytes.Length; i++)
+            {
+                bytes[i] = Convert.ToByte(hexString.Substring(i * 2, 2), 16);
+            }
+            return bytes;
+        }
+        
+        /// <summary>
+        /// 바이트 배열을 헥스 문자열로 변환
+        /// </summary>
+        /// <param name="bytes">바이트 배열</param>
+        /// <returns>헥스 문자열</returns>
+        public static string ToHexString(byte[] bytes)
+        {
+            return BitConverter.ToString(bytes).Replace("-", "");
+        }
+    }
+}