feat: download artist's top tracks with --limit flag

internal/downloader/options.go

@@ -16,12 +16,16 @@ type Options struct {
 	Quality string
 	Timeout time.Duration
 	BPM     bool
+	Limit   int
 }
 
 func (o *Options) Validate() error {
 	if !validQualities[o.Quality] {
 		return fmt.Errorf("invalid quality option: %s", o.Quality)
 	}
+	if o.Limit < 0 {
+		return fmt.Errorf("limit must be a non-negative integer")
+	}
 
 	return nil
 }