The navbar previously listed Item Types, Users, and Settings as three
separate top-level links. They now live under a single "Settings" entry
in the navbar, backed by one unified section with a left sub-nav (General,
Item Types, Users) shared across all three areas so it reads as one
settings panel rather than three disconnected admin pages.
- New templates/partials/settings_nav.html shared sub-nav, included by
settings.html, itemtypes_list/form.html, and users_list/form.html.
- Routes moved under /settings/: /item-types* -> /settings/item-types*,
/users* -> /settings/users*. All redirects, form actions, and the
item-type custom-field JS base action updated to match.