feature: implement cut and paste functionality (closes #8)

- Add CutStore (Zustand) to store cut node IDs, node data, edges, and source graph ID
- Update graphToDot() to render cut nodes with dashed grey style for visual indication
- Add 'Cut' action to node context menu; 'Paste' action to empty-area context menu
- Cut logic: captures selected nodes (or right-clicked node if none selected) plus
  all nodes reachable by outgoing edges from those nodes; clears selection afterwards
- Paste logic: adds cut nodes/edges to target graph, removes them from source graph
  in GraphsStore; pasting on the same graph as the cut source cancels the cut
- Graph re-renders automatically when cutNodeIds changes via useCutStore subscription
- Clear cut store on file load for consistency
- Add E2E tests covering: Cut menu visibility, Paste menu visibility, cut styling,
  linked-node inclusion, cross-subgraph paste, source-graph cleanup, same-graph cancel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Graphviz.tsx

@@ -1,6 +1,6 @@
 import type { GraphModel } from "./components/Graph";
 
-export function graphToDot(g: GraphModel): string {
+export function graphToDot(g: GraphModel, cutNodeIds: Set<string> = new Set()): string {
   // Directed graph, use neato layout so we can use pos attributes
   const lines = [];
   lines.push('digraph G {');
@@ -12,6 +12,9 @@ export function graphToDot(g: GraphModel): string {
     const attrs = [];
     attrs.push(`label=\"${n.label}\"`);
     attrs.push(`id=\"${n.id}\"`)
+    if (cutNodeIds.has(n.id)) {
+      attrs.push(`fillcolor="#d9d9d9"`, `style="filled,dashed"`);
+    }
     lines.push(`  \"${n.id}\" [${attrs.join(', ')}];`);
   }