refactor: migrate ES private fields to TypeScript private for Vue Proxy compatibility (#8440)

## Summary

Migrates ECMAScript private fields (`#`) to TypeScript private
(`private`) across LiteGraph to fix Vue Proxy reactivity
incompatibility.

## Problem

ES private fields (`#field`) are incompatible with Vue's Proxy-based
reactivity system - accessing `#field` through a Proxy throws
`TypeError: Cannot read private member from an object whose class did
not declare it`.

## Solution

- Converted all `#field` to `private _field` across 10 phases
- Added `toJSON()` methods to `LGraph`, `NodeSlot`, `NodeInputSlot`, and
`NodeOutputSlot` to prevent circular reference errors during
serialization (TypeScript private fields are visible to `JSON.stringify`
unlike true ES private fields)
- Made `DragAndScale.element.data` non-enumerable to break canvas
circular reference chain

## Testing

- All 4027 unit tests pass
- Added 9 new serialization tests to catch future circular reference
issues
- Browser tests (undo/redo, save workflows) verified working

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8440-refactor-migrate-ES-private-fields-to-TypeScript-private-for-Vue-Proxy-compatibility-2f76d73d365081a3bd82d429a3e0fcb7)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

src/extensions/core/groupNode.ts

@@ -292,10 +292,10 @@ export class GroupNodeConfig {
       this.processNode(node, seenInputs, seenOutputs)
     }
 
-    for (const p of this.#convertedToProcess) {
+    for (const p of this._convertedToProcess) {
       p()
     }
-    this.#convertedToProcess = []
+    this._convertedToProcess = []
     if (!this.nodeDef) return
     await app.registerNodeDef(`${PREFIX}${SEPARATOR}` + this.name, this.nodeDef)
     useNodeDefStore().addNodeDef(this.nodeDef)
@@ -773,7 +773,7 @@ export class GroupNodeConfig {
     }
   }
 
-  #convertedToProcess: (() => void)[] = []
+  private _convertedToProcess: (() => void)[] = []
   processNodeInputs(
     node: GroupNodeData,
     seenInputs: Record<string, number>,
@@ -804,7 +804,7 @@ export class GroupNodeConfig {
     )
 
     // Converted inputs have to be processed after all other nodes as they'll be at the end of the list
-    this.#convertedToProcess.push(() =>
+    this._convertedToProcess.push(() =>
       this.processConvertedWidgets(
         inputs,
         node,