feat: add ContextMenu component for network diagram editor

Charcoal-styled context menu with action factories for node
and canvas variants. Viewport-clamped positioning, auto-dismiss
on click outside, escape, or scroll.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/network/ContextMenu.tsx

@@ -0,0 +1,106 @@
+import { useEffect, useRef } from 'react'
+import { Copy, CopyPlus, Trash2, ClipboardPaste, BoxSelect, Maximize2 } from 'lucide-react'
+import { cn } from '@/lib/utils'
+
+interface MenuAction {
+  label: string
+  icon: React.ElementType
+  shortcut: string
+  onClick: () => void
+  disabled?: boolean
+}
+
+interface ContextMenuProps {
+  position: { x: number; y: number }
+  actions: MenuAction[]
+  onClose: () => void
+}
+
+export function ContextMenu({ position, actions, onClose }: ContextMenuProps) {
+  const menuRef = useRef<HTMLDivElement>(null)
+
+  const clampedPosition = { ...position }
+  if (typeof window !== 'undefined') {
+    const menuWidth = 192
+    const menuHeight = actions.length * 36 + 8
+    if (clampedPosition.x + menuWidth > window.innerWidth) {
+      clampedPosition.x = window.innerWidth - menuWidth - 8
+    }
+    if (clampedPosition.y + menuHeight > window.innerHeight) {
+      clampedPosition.y = window.innerHeight - menuHeight - 8
+    }
+  }
+
+  useEffect(() => {
+    const handleClickOutside = (e: MouseEvent) => {
+      if (menuRef.current && !menuRef.current.contains(e.target as HTMLElement)) {
+        onClose()
+      }
+    }
+    const handleEscape = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') onClose()
+    }
+    const handleScroll = () => onClose()
+
+    document.addEventListener('mousedown', handleClickOutside)
+    document.addEventListener('keydown', handleEscape)
+    document.addEventListener('scroll', handleScroll, true)
+    return () => {
+      document.removeEventListener('mousedown', handleClickOutside)
+      document.removeEventListener('keydown', handleEscape)
+      document.removeEventListener('scroll', handleScroll, true)
+    }
+  }, [onClose])
+
+  return (
+    <div
+      ref={menuRef}
+      className="fixed z-50 w-48 rounded-lg border border-default bg-card py-1"
+      style={{ left: clampedPosition.x, top: clampedPosition.y }}
+    >
+      {actions.map((action) => (
+        <button
+          key={action.label}
+          onClick={() => {
+            action.onClick()
+            onClose()
+          }}
+          disabled={action.disabled}
+          className={cn(
+            'flex w-full items-center gap-2 px-3 py-2 text-xs text-primary hover:bg-elevated',
+            action.disabled && 'opacity-40 pointer-events-none',
+          )}
+        >
+          <action.icon size={14} />
+          <span>{action.label}</span>
+          <span className="ml-auto text-[10px] text-muted-foreground">{action.shortcut}</span>
+        </button>
+      ))}
+    </div>
+  )
+}
+
+export function getNodeMenuActions(handlers: {
+  onCopy: () => void
+  onDuplicate: () => void
+  onDelete: () => void
+}): MenuAction[] {
+  return [
+    { label: 'Copy', icon: Copy, shortcut: 'Ctrl+C', onClick: handlers.onCopy },
+    { label: 'Duplicate', icon: CopyPlus, shortcut: 'Ctrl+D', onClick: handlers.onDuplicate },
+    { label: 'Delete', icon: Trash2, shortcut: 'Del', onClick: handlers.onDelete },
+  ]
+}
+
+export function getCanvasMenuActions(handlers: {
+  onPaste: () => void
+  onSelectAll: () => void
+  onFitView: () => void
+  hasClipboard: boolean
+}): MenuAction[] {
+  return [
+    { label: 'Paste', icon: ClipboardPaste, shortcut: 'Ctrl+V', onClick: handlers.onPaste, disabled: !handlers.hasClipboard },
+    { label: 'Select All', icon: BoxSelect, shortcut: 'Ctrl+A', onClick: handlers.onSelectAll },
+    { label: 'Fit View', icon: Maximize2, shortcut: '⌘⇧F', onClick: handlers.onFitView },
+  ]
+}