feat: implement full admin panel with dashboard, user management, and platform settings

Adds complete super_admin panel with 9 pages and account owner categories page.
Backend includes 5 new DB tables, ~25 API endpoints, settings manager with
in-memory cache, and 29 integration tests. Frontend includes reusable admin
components (DataTable, Pagination, ActionMenu, etc.) with code-split lazy loading.

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

frontend/src/components/admin/EmptyState.tsx

@@ -0,0 +1,25 @@
+import type { ReactNode } from 'react'
+import { cn } from '@/lib/utils'
+
+interface EmptyStateProps {
+  icon?: ReactNode
+  title: string
+  description?: string
+  action?: ReactNode
+  className?: string
+}
+
+export function EmptyState({ icon, title, description, action, className }: EmptyStateProps) {
+  return (
+    <div className={cn('flex flex-col items-center justify-center py-12 text-center', className)}>
+      {icon && <div className="mb-4 text-muted-foreground">{icon}</div>}
+      <h3 className="text-lg font-semibold text-foreground">{title}</h3>
+      {description && (
+        <p className="mt-1 max-w-sm text-sm text-muted-foreground">{description}</p>
+      )}
+      {action && <div className="mt-4">{action}</div>}
+    </div>
+  )
+}
+
+export default EmptyState