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/extensions/core/index.ts

@@ -19,6 +19,7 @@ if (!isCloud) {
   await import('./nodeTemplates')
 }
 import './noteNode'
+import './painter'
 import './previewAny'
 import './rerouteNode'
 import './saveImageExtraOutput'

src/extensions/core/painter.ts

@@ -0,0 +1,22 @@
+import { useExtensionService } from '@/services/extensionService'
+
+const HIDDEN_WIDGETS = new Set(['width', 'height', 'bg_color'])
+
+useExtensionService().registerExtension({
+  name: 'Comfy.Painter',
+
+  nodeCreated(node) {
+    if (node.constructor.comfyClass !== 'Painter') return
+
+    const [oldWidth, oldHeight] = node.size
+    node.setSize([Math.max(oldWidth, 450), Math.max(oldHeight, 550)])
+
+    node.hideOutputImages = true
+
+    for (const widget of node.widgets ?? []) {
+      if (HIDDEN_WIDGETS.has(widget.name)) {
+        widget.hidden = true
+      }
+    }
+  }
+})