A folder made in a cases directory is already a case (no scaffold), which Link
Existing then refuses by name and which deleting would take the linked folder
with. The route refuses a cases directory as the parent (the shared one and the
caller's own), a leading tilde in the name, and maps a vanished parent to 404.
Docs now say every name it accepts is one Link Existing accepts, and the wiki
describes the button.
The name rule now matches safePathSchema (letters, digits, space and _ - . ~,
no .., no leading dot, at most 255 bytes) so every folder it makes can be
linked. A file as the parent is a 400, errno text is no longer returned,
showHidden is forwarded like browse, and Create is disabled while the request
runs. Adds multi-user scoping tests.
The picker could only choose an existing folder. POST /api/filesystem/mkdir
creates one folder inside a folder the picker may browse (same roots,
ownership and blocked-tree rules as browse), and PathPicker gains an opt-in
allowNewFolder that adds a New folder button and selects the result.
Devvyn