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/layout/AppLayout.tsx

@@ -52,7 +52,7 @@ export function AppLayout() {
     { path: '/sessions', label: 'Sessions' },
     { path: '/account', label: 'Account' },
     { path: '/settings', label: 'Settings' },
-    ...(isSuperAdmin ? [{ path: '/admin/categories', label: 'Admin: Categories' }] : []),
+    ...(isSuperAdmin ? [{ path: '/admin', label: 'Admin Panel' }] : []),
   ]
 
   return (