fix: create parent dirs for downloads with subdirectory paths

Some archive.org file names include directory prefixes
(e.g. "ZERO-G - Deepest India/vocal.iso"). MkdirAll only
created the output root, not intermediate directories.
Added filepath.Dir(MkdirAll) in both downloadFile (CLI)
and handleAPIDownload (web).

main.go

@@ -330,6 +330,8 @@ func cmdDownload(args []string) {
 }
 
 func downloadFile(job downloadJob) (int64, error) {
+	os.MkdirAll(filepath.Dir(job.OutPath), 0755)
+
 	// Check if file already exists and has the expected size
 	if fi, err := os.Stat(job.OutPath); err == nil {
 		if job.Size > 0 && fi.Size() == job.Size {