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/pages/settings.html

@@ -1,6 +1,11 @@
 {{define "content"}}
-<div class="container-fluid p-3" style="max-width: 800px">
+<div class="container-fluid p-3">
   <h4 class="mb-3">Settings</h4>
+  <div class="row g-3">
+    <div class="col-md-3 col-lg-2">
+      {{template "settings_nav" "general"}}
+    </div>
+    <div class="col-md-9 col-lg-10" style="max-width: 800px">
 
   <div class="card mb-4">
     <div class="card-header py-2 fw-semibold">General</div>
@@ -82,5 +87,8 @@
       </form>
     </div>
   </div>
+
+    </div>
+  </div>
 </div>
 {{end}}