Fills in the 35 `sidebar.*` placeholders the folder-sidebar feature series
left behind — view options (tree/list, empty folders), folder creation,
the delete confirmation modal and its result toasts, and folder
renaming, including the undo copy and the "deleting a folder never
cascades over model files" rule the backend enforces.
All nine locales are translated, so no `[TODO: Translate]` placeholder
remains anywhere: the state §7 of the translation guidelines describes
holds again. Terminology reuses the existing §2 maps (folder, model
root, sidebar) with tree/list view added, recorded in a new §2
subsection; the stale leaf-count in the header is refreshed too.
Will Miao
3b54a13cae