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/index.ts

@@ -0,0 +1,9 @@
+export { DataTable, type Column } from './DataTable'
+export { Pagination } from './Pagination'
+export { ActionMenu, type ActionMenuItem } from './ActionMenu'
+export { StatusBadge } from './StatusBadge'
+export { EmptyState } from './EmptyState'
+export { SearchInput } from './SearchInput'
+export { PageHeader } from './PageHeader'
+export { AdminLayout } from './AdminLayout'
+export { AdminSidebar } from './AdminSidebar'