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.
- GetAllTaskFiles: retrieve all files attached to a task
- CreateTaskFile: upload file with base64 encoding
- DownloadTaskFile: download file with base64 decoding
- RemoveTaskFile: delete a task file
- TaskScope.GetFiles and UploadFile for fluent API
- Automatic base64 encoding/decoding for file content
- Comprehensive test coverage
