feat(helpdesk): add Domains list page with BC OData contract type enrichment

Add a new "Domains" page to the helpdesk module that fetches domain data
from the FS_Contract_Domains OData endpoint and enriches each domain with
its contract type (PI_Header_Type) by joining with FS_Contract_Lines_Test
where Type = 'Domain'.

New files:
- domains/index().php, index(default).phtml, filter-schema.php — list page
- domains-data().php — data endpoint with PHP-side filtering/sorting
- helpdesk-domains-index.js — Grid.js via initStandardListPage()

Gateway additions:
- listDomains() — fetch all contract domains
- listDomainContractLines() — fetch domain-type contract lines for type lookup

Sidebar restructured into three collapsible groups (Lookup, Monitoring,
Administration) with per-group icons and color coding, matching the core
admin panel pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/layout/app-sidebar.css

@@ -75,6 +75,17 @@
     --app-nav-icon-color: #6b7280;
   }
 
+  /* Helpdesk module groups */
+  .app-sidebar-admin-group [data-details-key="helpdesk-lookup"] {
+    --app-nav-icon-color: #2563eb;
+  }
+  .app-sidebar-admin-group [data-details-key="helpdesk-monitoring"] {
+    --app-nav-icon-color: #059669;
+  }
+  .app-sidebar-admin-group [data-details-key="helpdesk-administration"] {
+    --app-nav-icon-color: #6b7280;
+  }
+
   aside.app-sidebar li:has(> .app-empty-state) {
     padding-inline: 19px;
   }