feature: add Create Node action for orphaned node creation

Right-clicking on empty graph area (outside any node or edge) now shows
a context menu with a single 'Create Node' option. Clicking it adds a new
node with a random label and no edges, allowing the user to wire it up
manually afterwards.

- NodeContext gains isEmptyArea flag to distinguish empty-area from node
  right-clicks
- Container div contextmenu handler covers the white space below the SVG;
  SVG-level handler covers empty space within the rendered graph
- node contextmenu handler calls stopPropagation so the SVG handler never
  fires for node clicks; SVG handler also guards via target.closest check
- NodeContextMenu renders emptyAreaItems (Create Node only) vs nodeItems
  (Rename / Subgraph / Remove) based on the flag
- randomWordList extracted as a named export so NodeContextMenu can reuse
  the same word bank
- Three new Playwright e2e tests cover: empty-area menu shows only Create
  Node, created node is orphaned (no new edges), node right-click does not
  show Create Node

src/components/NodeContextMenu.tsx

@@ -1,11 +1,11 @@
 import { Dropdown, type MenuProps } from "antd";
-import { defaultGraph, graphContext, GraphModel, type EdgeModel, type NodeContext } from "./Graph";
+import { defaultGraph, graphContext, GraphModel, randomWordList, type EdgeModel, type NodeContext } from "./Graph";
 import { useContext } from "react";
 import { cloneDeep } from "lodash";
 import { useGraphsStore } from "../stores/GraphsStore";
 import { useGraphLayersTreeStore } from "../stores/TreeStore";
 
-const items: MenuProps['items'] = [
+const nodeItems: MenuProps['items'] = [
   {
     key: 'rename',
     label: 'Rename',
@@ -23,6 +23,13 @@ const items: MenuProps['items'] = [
   }
 ]
 
+const emptyAreaItems: MenuProps['items'] = [
+  {
+    key: 'create',
+    label: 'Create Node',
+  }
+]
+
 export default function NodeContextMenu({
   nodeContext,
   contextMenuOpened,
@@ -38,6 +45,7 @@ export default function NodeContextMenu({
     return;
   }
 
+  const items = nodeContext.isEmptyArea ? emptyAreaItems : nodeItems;
   const graphContextValue = useContext(graphContext)!;
   const graphsById = useGraphsStore((s) => (s as { graphsById: Map<string, GraphModel> }).graphsById);
   const addTreeNode = useGraphLayersTreeStore(store => store.add);
@@ -60,6 +68,11 @@ export default function NodeContextMenu({
         removeTreeNode(nodeContext.nodeId);
         break;
       }
+      case 'create': {
+        const id = crypto.randomUUID();
+        graphContextValue.setGraph(prev => ({ ...prev, nodes: [...prev.nodes, { id, label: randomWordList(1)[0] }] }));
+        break;
+      }
       case 'subgraph': {
         graphsById.set(graphContextValue.graphId, cloneDeep(graphContextValue.graph));
         graphContextValue.selectGraphId(nodeContext.nodeId);