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/users_list.html

@@ -1,24 +1,32 @@
 {{define "content"}}
 <div class="container-fluid p-3">
-  <div class="d-flex align-items-center mb-3">
-    <h4 class="mb-0">Users</h4>
-    <a href="/users/new" class="btn btn-primary btn-sm ms-auto"><i class="bi bi-person-plus"></i> New User</a>
-  </div>
-  <div class="table-responsive">
-    <table class="table table-hover align-middle">
-      <thead><tr><th>Username</th><th>Role</th><th>Status</th><th>Created</th><th></th></tr></thead>
-      <tbody>
-        {{range .Users}}
-        <tr>
-          <td><a href="/users/{{.ID}}/edit">{{.Username}}</a></td>
-          <td>{{if .IsAdmin}}<span class="badge text-bg-primary">Admin</span>{{else}}<span class="badge text-bg-secondary">User</span>{{end}}</td>
-          <td>{{if .Enabled}}<span class="badge text-bg-success">Enabled</span>{{else}}<span class="badge text-bg-secondary">Disabled</span>{{end}}</td>
-          <td>{{fmtDateShort .CreatedAt}}</td>
-          <td class="text-end"><a href="/users/{{.ID}}/edit" class="btn btn-sm btn-outline-secondary">Edit</a></td>
-        </tr>
-        {{end}}
-      </tbody>
-    </table>
+  <h4 class="mb-3">Settings</h4>
+  <div class="row g-3">
+    <div class="col-md-3 col-lg-2">
+      {{template "settings_nav" "users"}}
+    </div>
+    <div class="col-md-9 col-lg-10">
+      <div class="d-flex align-items-center mb-3">
+        <h5 class="mb-0">Users</h5>
+        <a href="/settings/users/new" class="btn btn-primary btn-sm ms-auto"><i class="bi bi-person-plus"></i> New User</a>
+      </div>
+      <div class="table-responsive">
+        <table class="table table-hover align-middle">
+          <thead><tr><th>Username</th><th>Role</th><th>Status</th><th>Created</th><th></th></tr></thead>
+          <tbody>
+            {{range .Users}}
+            <tr>
+              <td><a href="/settings/users/{{.ID}}/edit">{{.Username}}</a></td>
+              <td>{{if .IsAdmin}}<span class="badge text-bg-primary">Admin</span>{{else}}<span class="badge text-bg-secondary">User</span>{{end}}</td>
+              <td>{{if .Enabled}}<span class="badge text-bg-success">Enabled</span>{{else}}<span class="badge text-bg-secondary">Disabled</span>{{end}}</td>
+              <td>{{fmtDateShort .CreatedAt}}</td>
+              <td class="text-end"><a href="/settings/users/{{.ID}}/edit" class="btn btn-sm btn-outline-secondary">Edit</a></td>
+            </tr>
+            {{end}}
+          </tbody>
+        </table>
+      </div>
+    </div>
   </div>
 </div>
 {{end}}