fix: replace JSON deep-clone with clonePreservingStrings in clipboard and snapshot

copySelectedNodes, pasteNodes, and AI captureSnapshot were still using
JSON.parse(JSON.stringify(...)) which duplicates multi-MB base64 blobs.
Switched to clonePreservingStrings which shares immutable string refs.
Also clarified docstring re: toJSON not being called.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/store/undoHistory.ts

@@ -57,9 +57,13 @@ export class UndoManager {
  * snapshots, but strings (immutable in JS) are returned by reference.
  * This avoids duplicating multi-MB base64 blobs across undo history.
  *
- * Matches JSON.parse(JSON.stringify()) semantics:
+ * Matches JSON.parse(JSON.stringify()) semantics for plain JSON-like
+ * objects/arrays:
  *  - `undefined` values are dropped from objects, become `null` in arrays
  *  - functions are dropped from objects, become `null` in arrays
+ *
+ * Does NOT call toJSON() on objects. Objects with custom toJSON methods
+ * are treated as plain objects (their enumerable own properties are cloned).
  */
 export function clonePreservingStrings<T>(value: T): T {
   if (value === null || typeof value !== "object") {

src/store/workflowStore.ts

@@ -907,8 +907,8 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
     );
 
     // Deep clone the nodes and edges to avoid reference issues
-    const clonedNodes = JSON.parse(JSON.stringify(selectedNodes)) as WorkflowNode[];
+    const clonedNodes = clonePreservingStrings(selectedNodes) as WorkflowNode[];
-    const clonedEdges = JSON.parse(JSON.stringify(connectedEdges)) as WorkflowEdge[];
+    const clonedEdges = clonePreservingStrings(connectedEdges) as WorkflowEdge[];
 
     set({ clipboard: { nodes: clonedNodes, edges: clonedEdges } });
   },
@@ -946,7 +946,7 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
         width: undefined,
         height: undefined,
         measured: undefined,
-        data: JSON.parse(JSON.stringify(node.data)),
+        data: clonePreservingStrings(node.data),
       };
     });
 
@@ -2536,12 +2536,12 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
   captureSnapshot: () => {
     const state = get();
     // Deep copy the current workflow state to avoid reference sharing
-    const snapshot = {
+    const snapshot = clonePreservingStrings({
-      nodes: JSON.parse(JSON.stringify(state.nodes)),
+      nodes: state.nodes,
-      edges: JSON.parse(JSON.stringify(state.edges)),
+      edges: state.edges,
-      groups: JSON.parse(JSON.stringify(state.groups)),
+      groups: state.groups,
       edgeStyle: state.edgeStyle,
-    };
+    });
     set({
       previousWorkflowSnapshot: snapshot,
       manualChangeCount: 0,