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).

serve.go

@@ -189,6 +189,8 @@ func handleAPIDownload(w http.ResponseWriter, r *http.Request) {
 		jobIDs = append(jobIDs, jobID)
 
 		go func(j downloadJob, s *downloadState) {
+			os.MkdirAll(filepath.Dir(j.OutPath), 0755)
+
 			client := &http.Client{Timeout: 0}
 			req, _ := http.NewRequest("GET", j.URL, nil)
 			req.Header.Set("User-Agent", userAgent)