Fix empty filename when Content-Disposition header is missing in /dl command

Add filenameFromURL helper to extract filename from URL path when
Content-Disposition header is empty or not present. This fixes the issue
where direct link downloads fail due to empty filename.

Co-authored-by: krau <71133316+krau@users.noreply.github.com>

core/tasks/directlinks/execute.go

@@ -45,10 +45,14 @@ func (t *Task) Execute(ctx context.Context) error {
 			fetchedTotalBytes.Add(resp.ContentLength)
 			file.Size = resp.ContentLength
 			if name := resp.Header.Get("Content-Disposition"); name != "" {
-				// Set file name
+				// Set file name from Content-Disposition header
 				filename := parseFilename(name)
 				file.Name = filename
 			}
+			// Fallback: extract filename from URL if Content-Disposition is empty
+			if file.Name == "" {
+				file.Name = filenameFromURL(file.URL)
+			}
 
 			return nil
 		})