feat: wire context menu and keyboard shortcuts into diagram editor

Right-click context menus for nodes (copy/duplicate/delete) and
canvas (paste/select-all/fit-view). Right-click selects the node
per spec. serializeNodes now handles group nodes correctly.

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

frontend/src/components/network/NetworkCanvas.tsx

@@ -26,6 +26,8 @@ interface NetworkCanvasProps {
   onDragOver: (event: React.DragEvent) => void
   onDragLeave?: (event: React.DragEvent) => void
   isDragOver?: boolean
+  onNodeContextMenu?: (event: React.MouseEvent, node: Node) => void
+  onPaneContextMenu?: (event: MouseEvent | React.MouseEvent) => void
 }
 
 export function NetworkCanvas({
@@ -40,6 +42,8 @@ export function NetworkCanvas({
   onDragOver,
   onDragLeave,
   isDragOver,
+  onNodeContextMenu,
+  onPaneContextMenu,
 }: NetworkCanvasProps) {
   const handleSelectionChange = useCallback(({ nodes: selectedNodes, edges: selectedEdges }: { nodes: Node[]; edges: Edge[] }) => {
     if (selectedNodes.length === 1) {
@@ -71,6 +75,8 @@ export function NetworkCanvas({
         onPaneClick={handlePaneClick}
         onDrop={onDrop}
         onDragOver={onDragOver}
+        onNodeContextMenu={onNodeContextMenu}
+        onPaneContextMenu={onPaneContextMenu}
         nodeTypes={nodeTypes}
         edgeTypes={edgeTypes}
         defaultEdgeOptions={{ type: 'connection' }}