Cleanup: Litegraph/Vue synchronization work (#5789)

## Summary

Cleanup and fixes to the existing syncing logic.

## Review Focus

This is probably enough to review and test now.

Main things that should still work: 
- moving nodes around
- adding new ones
- switching back and forth between Vue and Litegraph

Let me know if you find any bugs that weren't already present there.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5789-WIP-Litegraph-Vue-synchronization-work-27a6d73d3650811682cacacb82367b9e)
by [Unito](https://www.unito.io)

src/composables/useCanvasDrop.ts

@@ -14,7 +14,7 @@ import { useModelToNodeStore } from '@/stores/modelToNodeStore'
 import { ComfyNodeDefImpl } from '@/stores/nodeDefStore'
 import type { RenderedTreeExplorerNode } from '@/types/treeExplorerTypes'
 
-export const useCanvasDrop = (canvasRef: Ref<HTMLCanvasElement>) => {
+export const useCanvasDrop = (canvasRef: Ref<HTMLCanvasElement | null>) => {
   const modelToNodeStore = useModelToNodeStore()
   const litegraphService = useLitegraphService()
   const workflowService = useWorkflowService()