feat(ui): add system settings to sidebar and unhide frontend theme switcher

- Add System Settings entry to Admin sidebar group with Settings icon
- Register /system-settings URL mapping in sidebar config
- Remove 3-click hidden unlock from Frontend Theme setting, making it
  directly visible in System Information section

web/default/src/hooks/use-sidebar-data.ts

@@ -14,6 +14,7 @@ import {
   MessageSquare,
   CreditCard,
   ListTodo,
+  Settings,
 } from 'lucide-react'
 import { useTranslation } from 'react-i18next'
 import { WORKSPACE_IDS } from '@/components/layout/lib/workspace-registry'
@@ -120,6 +121,12 @@ export function useSidebarData(): SidebarData {
             url: '/subscriptions',
             icon: CreditCard,
           },
+          {
+            title: t('System Settings'),
+            url: '/system-settings/general',
+            activeUrls: ['/system-settings'],
+            icon: Settings,
+          },
         ],
       },
     ],