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/renderer/extensions/vueNodes/widgets/composables/usePainterWidget.ts

@@ -0,0 +1,16 @@
+import type { LGraphNode } from '@/lib/litegraph/src/litegraph'
+import type { IBaseWidget } from '@/lib/litegraph/src/types/widgets'
+import type { InputSpec as InputSpecV2 } from '@/schemas/nodeDef/nodeDefSchemaV2'
+import type { ComfyWidgetConstructorV2 } from '@/scripts/widgets'
+
+export const usePainterWidget = (): ComfyWidgetConstructorV2 => {
+  return (node: LGraphNode, inputSpec: InputSpecV2): IBaseWidget => {
+    return node.addWidget(
+      'painter',
+      inputSpec.name,
+      (inputSpec.default as string) ?? '',
+      null,
+      { serialize: true, canvasOnly: false }
+    ) as IBaseWidget
+  }
+}

src/renderer/extensions/vueNodes/widgets/registry/widgetRegistry.ts

@@ -60,6 +60,9 @@ const WidgetBoundingBox = defineAsyncComponent(
 const WidgetCurve = defineAsyncComponent(
   () => import('@/components/curve/WidgetCurve.vue')
 )
+const WidgetPainter = defineAsyncComponent(
+  () => import('@/components/painter/WidgetPainter.vue')
+)
 
 export const FOR_TESTING = {
   WidgetButton,
@@ -186,6 +189,14 @@ const coreWidgetDefinitions: Array<[string, WidgetDefinition]> = [
       aliases: ['CURVE'],
       essential: false
     }
+  ],
+  [
+    'painter',
+    {
+      component: WidgetPainter,
+      aliases: ['PAINTER'],
+      essential: false
+    }
   ]
 ]
 
@@ -217,7 +228,13 @@ export const shouldRenderAsVue = (widget: Partial<SafeWidgetData>): boolean => {
   return !widget.options?.canvasOnly && !!widget.type
 }
 
-const EXPANDING_TYPES = ['textarea', 'markdown', 'load3D', 'curve'] as const
+const EXPANDING_TYPES = [
+  'textarea',
+  'markdown',
+  'load3D',
+  'curve',
+  'painter'
+] as const
 
 export function shouldExpand(type: string): boolean {
   const canonicalType = getCanonicalType(type)