fix: restore mask editor compatibility with Impact-Pack plugin (#7762)

## Summary

- Skip widget filenames starting with '$' (internal reference format)
- Add deprecated ComfyApp.open_maskeditor for plugin compatibility
- Register MaskEditor button in ClipspaceDialog

fix https://github.com/Comfy-Org/ComfyUI_frontend/issues/7665 and 
https://github.com/ltdrdata/ComfyUI-Impact-Pack/issues/1165
https://github.com/ltdrdata/ComfyUI-Impact-Pack/issues/1158
https://github.com/ltdrdata/ComfyUI-Impact-Pack/issues/1157

## Screenshots (if applicable)
before


https://github.com/user-attachments/assets/356dd920-8b64-40f4-8839-90b955ffafc3

after


https://github.com/user-attachments/assets/dff9abcd-530a-4995-bb4b-51f408d4eca9

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7762-fix-restore-mask-editor-compatibility-with-Impact-Pack-plugin-2d46d73d36508199bdaecbd5ba838483)
by [Unito](https://www.unito.io)

src/composables/maskeditor/useMaskEditorLoader.ts

@@ -104,7 +104,8 @@ export function useMaskEditorLoader() {
       // If we have a widget filename, we should prioritize it over the node image
       // because the node image might be stale (e.g. from a previous save)
       // while the widget value reflects the current selection.
-      if (widgetFilename) {
+      // Skip internal reference formats (e.g. "$35-0" used by some plugins like Impact-Pack)
+      if (widgetFilename && !widgetFilename.startsWith('$')) {
         try {
           // Parse the widget value which might be in format "subfolder/filename [type]" or just "filename"
           let filename = widgetFilename