fix: unpacking a missing node causes it to disappear (#7341)

## Summary

Fixes the issue where unpacking a subgraph containing missing nodes
causes those nodes to disappear. Missing nodes are now automatically
restored as placeholder nodes that preserve their original data,
allowing them to be recovered when the node types are installed later.

## Changes

- **What**: 
- Modified `multiClone()` to preserve missing nodes as serialized data
when creating subgraphs
- Added `skipMissingNodes` option to `unpackSubgraph()` method to
restore missing nodes as placeholder nodes instead of throwing errors
- Updated `useSubgraphOperations.unpackSubgraph()` to automatically
restore missing nodes as placeholders (removed confirmation dialog)
- Replaced deprecated `LiteGraph.cloneObject()` with `structuredClone()`
  - Removed unused i18n keys and debugging logs

## Review Focus

- **Placeholder node restoration**: Missing nodes are restored using the
same mechanism as `LGraph.configure()` (creating `LGraphNode` with
`last_serialization` and `has_errors` flags). This ensures compatibility
with the existing missing node manager.
- **Performance**: Optimized `getMissingNodeTypes()` to check
`registered_node_types` first before attempting node creation, and uses
Set for O(1) duplicate checking.
- **Data preservation**: Missing nodes preserve their original type,
title, and serialized data in `last_serialization`, allowing automatic
recovery when node types are installed.
- **Backward compatibility**: The `skipMissingNodes` option defaults to
`false`, maintaining original behavior for other code paths. Only the
UI-level `unpackSubgraph()` always uses `skipMissingNodes: true`.

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #7279 -->

## Demo

Before:


https://github.com/user-attachments/assets/e0327d05-802d-4a64-a9db-4d174e185d82

After:


https://github.com/user-attachments/assets/37ab3140-0ada-480e-b9d5-fef8856f8b27

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7341-fix-unpacking-a-missing-node-causes-it-to-disappear-2c66d73d36508151ac6be70a7b2bc56d)
by [Unito](https://www.unito.io)

src/services/litegraphService.ts

@@ -2,6 +2,7 @@ import _ from 'es-toolkit/compat'
 
 import { downloadFile } from '@/base/common/downloadUtil'
 import { useSelectedLiteGraphItems } from '@/composables/canvas/useSelectedLiteGraphItems'
+import { useSubgraphOperations } from '@/composables/graph/useSubgraphOperations'
 import { useNodeAnimatedImage } from '@/composables/node/useNodeAnimatedImage'
 import { useNodeCanvasImagePreview } from '@/composables/node/useNodeCanvasImagePreview'
 import { useNodeImage, useNodeVideo } from '@/composables/node/useNodeImage'
@@ -661,8 +662,8 @@ export const useLitegraphService = () => {
           {
             content: 'Unpack Subgraph',
             callback: () => {
-              useNodeOutputStore().revokeSubgraphPreviews(this)
-              this.graph.unpackSubgraph(this)
+              const { unpackSubgraph } = useSubgraphOperations()
+              unpackSubgraph()
             }
           }
         )