Add IntOrFalse type to handle polymorphic API responses

Kanboard API returns int (ID) on success and false (bool) on failure
for create operations. Add IntOrFalse type that handles both cases
and update CreateTask, CreateComment, CreateTag, CreateTaskFile, and
CreateTaskLink to use it.

files.go

@@ -29,7 +29,7 @@ func (c *Client) CreateTaskFile(ctx context.Context, projectID, taskID int, file
 		"blob":       base64.StdEncoding.EncodeToString(content),
 	}
 
-	var result int
+	var result IntOrFalse
 	if err := c.call(ctx, "createTaskFile", params, &result); err != nil {
 		return 0, fmt.Errorf("createTaskFile: %w", err)
 	}
@@ -38,7 +38,7 @@ func (c *Client) CreateTaskFile(ctx context.Context, projectID, taskID int, file
 		return 0, fmt.Errorf("createTaskFile: failed to upload file")
 	}
 
-	return result, nil
+	return int(result), nil
 }
 
 // DownloadTaskFile downloads a file's content by its ID.