fix using shift modifier to (de-)select Vue nodes (#5714)

## Summary

Fixes https://github.com/Comfy-Org/ComfyUI_frontend/issues/5688 by
adding shift modifier support for multi-selecting Vue nodes, enabling
standard shift+click selection behavior alongside existing
ctrl/cmd+click.

## Changes

- **What**: Updated Vue node event handlers to include `event.shiftKey`
in multi-select logic
- **Testing**: Added browser tests for both ctrl and shift modifier
selection behaviors

## Review Focus

Multi-select behavior consistency across different input modifiers and
platform compatibility (Windows/Mac/Linux shift key handling).

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5714-fix-using-shift-modifier-to-de-select-Vue-nodes-2756d73d365081bcb5e0fe80eacdb2f0)
by [Unito](https://www.unito.io)

src/renderer/extensions/vueNodes/composables/useNodeEventHandlers.ts

@@ -38,7 +38,7 @@ function useNodeEventHandlersIndividual() {
     const node = nodeManager.value.getNode(nodeData.id)
     if (!node) return
 
-    const isMultiSelect = event.ctrlKey || event.metaKey
+    const isMultiSelect = event.ctrlKey || event.metaKey || event.shiftKey
 
     if (isMultiSelect) {
       // Ctrl/Cmd+click -> toggle selection