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/AdminSidebar.tsx

@@ -0,0 +1,79 @@
+import { Link, useLocation } from 'react-router-dom'
+import {
+  LayoutDashboard,
+  Users,
+  Ticket,
+  FileText,
+  Gauge,
+  ToggleLeft,
+  Settings,
+  FolderTree,
+  ArrowLeft,
+} from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+const navItems = [
+  { path: '/admin', label: 'Dashboard', icon: LayoutDashboard, end: true },
+  { path: '/admin/users', label: 'Users', icon: Users },
+  { path: '/admin/invite-codes', label: 'Invite Codes', icon: Ticket },
+  { path: '/admin/audit-logs', label: 'Audit Logs', icon: FileText },
+  { path: '/admin/plan-limits', label: 'Plan Limits', icon: Gauge },
+  { path: '/admin/feature-flags', label: 'Feature Flags', icon: ToggleLeft },
+  { path: '/admin/settings', label: 'Settings', icon: Settings },
+  { path: '/admin/categories', label: 'Categories', icon: FolderTree },
+]
+
+interface AdminSidebarProps {
+  className?: string
+  onNavigate?: () => void
+}
+
+export function AdminSidebar({ className, onNavigate }: AdminSidebarProps) {
+  const location = useLocation()
+
+  const isActive = (path: string, end?: boolean) => {
+    if (end) return location.pathname === path
+    return location.pathname.startsWith(path)
+  }
+
+  return (
+    <aside className={cn('flex h-full flex-col', className)}>
+      <div className="p-4">
+        <h2 className="font-heading text-lg font-bold text-foreground">Admin Panel</h2>
+      </div>
+      <nav className="flex-1 space-y-1 px-3">
+        {navItems.map((item) => (
+          <Link
+            key={item.path}
+            to={item.path}
+            onClick={onNavigate}
+            className={cn(
+              'flex items-center gap-3 rounded-md px-3 py-2 text-sm font-medium transition-colors',
+              isActive(item.path, item.end)
+                ? 'bg-accent text-accent-foreground'
+                : 'text-muted-foreground hover:bg-accent hover:text-accent-foreground'
+            )}
+          >
+            <item.icon className="h-4 w-4" />
+            {item.label}
+          </Link>
+        ))}
+      </nav>
+      <div className="border-t border-border p-3">
+        <Link
+          to="/trees"
+          onClick={onNavigate}
+          className={cn(
+            'flex items-center gap-3 rounded-md px-3 py-2 text-sm font-medium',
+            'text-muted-foreground hover:bg-accent hover:text-accent-foreground'
+          )}
+        >
+          <ArrowLeft className="h-4 w-4" />
+          Back to App
+        </Link>
+      </div>
+    </aside>
+  )
+}
+
+export default AdminSidebar