Redefine the empty download filename template from a no-op to a bulk
revert: FilenameTemplateUseCase resolves the target from each model's
recorded original_file_name sidecar entry (skipping models without one),
which resolves follow-ups 1 and 2 with a single coherent semantic shared
by the download and bulk-apply paths.
Also replace the browser-native confirm() with a self-managed
confirmation modal (filenameTemplateConfirmModal) that stacks above the
settings modal, since ModalManager would close the settings modal when
opening a registered one.
Will Miao