feat: add Painter Node (#8521)

## Summary
Add PainterNode widget for freehand mask drawing directly on the canvas,
with brush/eraser tools, opacity, hardness, and background color
controls.

need BE changes https://github.com/Comfy-Org/ComfyUI/pull/12294

## Screenshots (if applicable)


https://github.com/user-attachments/assets/7222063a-0e40-40bb-b72e-b42c8984beb9



┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8521-feat-add-Painter-Node-2fa6d73d36508124ab2ede449a0cc67a)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/widgets/widgetMap.ts

@@ -21,6 +21,7 @@ import { FileUploadWidget } from './FileUploadWidget'
 import { GalleriaWidget } from './GalleriaWidget'
 import { GradientSliderWidget } from './GradientSliderWidget'
 import { ImageCompareWidget } from './ImageCompareWidget'
+import { PainterWidget } from './PainterWidget'
 import { ImageCropWidget } from './ImageCropWidget'
 import { KnobWidget } from './KnobWidget'
 import { LegacyWidget } from './LegacyWidget'
@@ -58,6 +59,7 @@ export type WidgetTypeMap = {
   imagecrop: ImageCropWidget
   boundingbox: BoundingBoxWidget
   curve: CurveWidget
+  painter: PainterWidget
   [key: string]: BaseWidget
 }
 
@@ -136,6 +138,8 @@ export function toConcreteWidget<TWidget extends IWidget | IBaseWidget>(
       return toClass(BoundingBoxWidget, narrowedWidget, node)
     case 'curve':
       return toClass(CurveWidget, narrowedWidget, node)
+    case 'painter':
+      return toClass(PainterWidget, narrowedWidget, node)
     default: {
       if (wrapLegacyWidgets) return toClass(LegacyWidget, widget, node)
     }