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/account/AccountLayout.tsx

@@ -0,0 +1,11 @@
+import { Outlet } from 'react-router-dom'
+
+export function AccountLayout() {
+  return (
+    <div className="container mx-auto max-w-screen-lg px-4 py-6">
+      <Outlet />
+    </div>
+  )
+}
+
+export default AccountLayout