Migrate checkbox template from GOV.UK to USWDS patterns

- Replace usaRadioLabel with usaLabel for checkbox labels
- Update hint classes from govuk-checkboxes__hint to usa-checkbox__label-description
- Change conditional reveal classes from govuk-specific to USWDS display-none utility
- Update template comments to reflect USWDS migration
- Add checkbox-specific routes to BackstopJS configuration for testing

This establishes a baseline for checkbox component migration with expected
spacing differences between GOV.UK and USWDS implementations.

urls.js

@@ -58,6 +58,19 @@ const routes = {
       label: 'Team Members',
       path: '/services/e6de050d-bd56-4f49-b44b-cb789c4f79fd/users',
     },
+    {
+      label: 'Invite User',
+      path: '/services/e6de050d-bd56-4f49-b44b-cb789c4f79fd/users/invite',
+    },
+    // Platform admin pages with checkboxes
+    {
+      label: 'Platform Admin Live Services',
+      path: '/platform-admin/live-services',
+    },
+    {
+      label: 'Platform Admin Trial Services',
+      path: '/platform-admin/trial-services',
+    },
   ],
 
   // Using Notify section