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/PageHeader.tsx

@@ -0,0 +1,25 @@
+import type { ReactNode } from 'react'
+import { cn } from '@/lib/utils'
+
+interface PageHeaderProps {
+  title: string
+  description?: string
+  action?: ReactNode
+  className?: string
+}
+
+export function PageHeader({ title, description, action, className }: PageHeaderProps) {
+  return (
+    <div className={cn('flex items-start justify-between gap-4', className)}>
+      <div>
+        <h1 className="font-heading text-2xl font-bold text-foreground">{title}</h1>
+        {description && (
+          <p className="mt-1 text-sm text-muted-foreground">{description}</p>
+        )}
+      </div>
+      {action && <div className="flex-shrink-0">{action}</div>}
+    </div>
+  )
+}
+
+export default PageHeader