Consolidate Item Types, Users, and Settings into one Settings section

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.

web/templates/layout.html

@@ -20,8 +20,6 @@
       <ul class="navbar-nav me-auto">
         <li class="nav-item"><a class="nav-link" href="/inventory">Inventory</a></li>
         {{if .User.IsAdmin}}
-        <li class="nav-item"><a class="nav-link" href="/item-types">Item Types</a></li>
-        <li class="nav-item"><a class="nav-link" href="/users">Users</a></li>
         <li class="nav-item"><a class="nav-link" href="/settings">Settings</a></li>
         {{end}}
       </ul>