fix: #I7VP0I 检测文件失败时，使用系统默认编码

src/main/java/org/jcnc/jnotepad/tool/EncodingDetector.java

@@ -60,7 +60,7 @@ public class EncodingDetector {
     public static Charset detectEncodingCharset(File file) {
         String charset = detectEncoding(file);
         if (charset.equals(UNKNOWN)) {
-            return StandardCharsets.UTF_8;
+            return Charset.defaultCharset();
         }
         return Charset.forName(charset);
     }