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/ActionMenu.tsx

@@ -0,0 +1,71 @@
+import { useState, useRef, useEffect, type ReactNode } from 'react'
+import { MoreHorizontal } from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+export interface ActionMenuItem {
+  label: string
+  icon?: ReactNode
+  onClick: () => void
+  destructive?: boolean
+  disabled?: boolean
+}
+
+interface ActionMenuProps {
+  items: ActionMenuItem[]
+}
+
+export function ActionMenu({ items }: ActionMenuProps) {
+  const [open, setOpen] = useState(false)
+  const ref = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    if (!open) return
+    const handler = (e: MouseEvent) => {
+      if (ref.current && !ref.current.contains(e.target as Node)) {
+        setOpen(false)
+      }
+    }
+    document.addEventListener('mousedown', handler)
+    return () => document.removeEventListener('mousedown', handler)
+  }, [open])
+
+  return (
+    <div className="relative" ref={ref}>
+      <button
+        onClick={() => setOpen(!open)}
+        className={cn(
+          'rounded-md p-1.5 text-muted-foreground transition-colors',
+          'hover:bg-accent hover:text-accent-foreground'
+        )}
+      >
+        <MoreHorizontal className="h-4 w-4" />
+      </button>
+      {open && (
+        <div className={cn(
+          'absolute right-0 top-full z-50 mt-1 min-w-[160px] rounded-md border border-border',
+          'bg-card py-1 shadow-lg animate-scale-in'
+        )}>
+          {items.map((item) => (
+            <button
+              key={item.label}
+              onClick={() => { item.onClick(); setOpen(false) }}
+              disabled={item.disabled}
+              className={cn(
+                'flex w-full items-center gap-2 px-3 py-2 text-sm transition-colors',
+                'disabled:opacity-50 disabled:pointer-events-none',
+                item.destructive
+                  ? 'text-destructive hover:bg-destructive/10'
+                  : 'text-foreground hover:bg-accent'
+              )}
+            >
+              {item.icon}
+              {item.label}
+            </button>
+          ))}
+        </div>
+      )}
+    </div>
+  )
+}
+
+export default ActionMenu