fix(vueNodes): propagate height to DOM widget children (#8821)

## Summary
DOM widgets using CSS background-image (e.g. KJNodes FastPreview)
collapsed to 0px height in vueNodes mode because background-image
doesn't contribute to intrinsic element size. Make WidgetDOM a flex
column container so mounted extension elements fill the available grid
row space.

## Screenshots (if applicable)
before 



https://github.com/user-attachments/assets/85de0295-1f7c-4142-ac15-1e813c823e3f


after


https://github.com/user-attachments/assets/824ab662-14cb-412d-93dd-97c0f549f992

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8821-fix-vueNodes-propagate-height-to-DOM-widget-children-3056d73d36508134926dc67336fd0d70)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/renderer/extensions/vueNodes/widgets/components/WidgetDOM.vue

@@ -34,5 +34,11 @@ onMounted(() => {
 whenever(() => !canvasStore.linearMode, mountWidgetElement)
 </script>
 <template>
-  <div ref="domEl" @pointerdown.stop @pointermove.stop @pointerup.stop />
+  <div
+    ref="domEl"
+    class="flex flex-col [&>*]:flex-1"
+    @pointerdown.stop
+    @pointermove.stop
+    @pointerup.stop
+  />
 </template>