feat: download single tracks (#5)

Introduces the `godeez download track <ID>` command, allowing users to download individual songs directly.

Single tracks are saved into a "Singles" folder by default to keep them organized. The README has been updated with documentation and examples for this new functionality.

internal/deezer/client.go

@@ -45,6 +45,8 @@ func (c *Client) FetchResource(ctx context.Context, resource Resource, id string
 		payload["alb_id"] = id
 	case *Artist:
 		payload["art_id"] = id
+	case *Track:
+		payload["sng_id"] = id
 	default:
 		return fmt.Errorf("unsupported resource type: %T", r)
 	}
@@ -84,6 +86,9 @@ func (c *Client) FetchResource(ctx context.Context, resource Resource, id string
 	if strings.Contains(string(body), `"DATA_ERROR":"artist::getData"`) {
 		return fmt.Errorf("invalid artist ID")
 	}
+	if strings.Contains(string(body), `"DATA_ERROR":"song::getData"`) {
+		return fmt.Errorf("invalid track ID")
+	}
 	if strings.Contains(string(body), `"results":{}`) {
 		return fmt.Errorf("unexpected response")
 	}