feat: add admin survey invites page with create and list

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/admin/AdminSidebar.tsx

@@ -8,6 +8,7 @@ import {
   ToggleLeft,
   Settings,
   FolderTree,
+  ClipboardList,
   ArrowLeft,
 } from 'lucide-react'
 import { cn } from '@/lib/utils'
@@ -21,6 +22,7 @@ const navItems = [
   { path: '/admin/feature-flags', label: 'Feature Flags', icon: ToggleLeft },
   { path: '/admin/settings', label: 'Settings', icon: Settings },
   { path: '/admin/categories', label: 'Categories', icon: FolderTree },
+  { path: '/admin/survey-invites', label: 'Survey Invites', icon: ClipboardList },
 ]
 
 interface AdminSidebarProps {