新增: 文件操作增强与预览扩展

- 文件剪贴板复制剪切粘贴，重名自动副本
- SFTP下载到本机与外部文件拖入，目录递归
- 传输进度浮动面板，transfer-progress事件链
- Drawio预览，Excel工作线程，冻结看门狗
- cmd与bat自写batch语法高亮，psm1等映射补齐

internal/filesystem/config.go

@@ -288,6 +288,8 @@ func getAllowedExtensions() map[string]bool {
 		".md":   true,
 		".json": true,
 		".xml":  true,
+		".drawio": true,
+		".dio":    true,
 		".html": true,
 		".css":  true,
 		".js":   true,
@@ -369,6 +371,8 @@ func getMIMETypeMapping() map[string]string {
 		".js":   "application/javascript",
 		".json": "application/json",
 		".xml":  "application/xml",
+		".drawio": "application/xml",
+		".dio":    "application/xml",
 		".md":   "text/markdown",
 		// 表格
 		".csv": "text/csv; charset=utf-8",