fix: restore widget.inputEl backward compatibility for custom nodes (#9759)

## Summary

Restores `widget.inputEl` assignment on STRING multiline widgets that
was removed in commit a7c211516 (PR #8594) when it was renamed to
`widget.element`. Custom nodes (e.g. comfyui-custom-scripts) rely on
`widget.inputEl` to call `addEventListener` or set `readOnly`.

- Fixes Comfy-Org/ComfyUI#12893

## Test plan

- Verify custom nodes that access `widget.inputEl` on STRING widgets
work correctly
- Verify `widget.element` still works as before

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/renderer/extensions/vueNodes/widgets/composables/useStringWidget.ts

@@ -40,6 +40,7 @@ function addMultilineWidget(
   })
 
   widget.element = inputEl
+  widget.inputEl = inputEl
   widget.options.minNodeSize = [400, 200]
 
   inputEl.addEventListener('input', (event) => {