Forward scroll unless focused (#6597)

## Summary

Forward wheel events to the canvas unless a wheel-capturing element is
focused, and ensure the Load3D scene becomes focusable on pointer
interaction so its wheel zoom/pan works after the user clicks into it.

## Changes

- **What**: gate wheel forwarding on focused capture elements; focus the
Load3D scene container on pointerdown to opt into wheel capture.
- **Dependencies**: none

## Review Focus

- Validate wheel forwarding behavior across focusable inputs vs.
non-focusable capture zones.
- Confirm Load3D zoom/pan only captures wheel after a user click (canvas
pan should still work when merely hovering).

## Screenshots (if applicable)

N/A

---------

Co-authored-by: Christian Byrne <cbyrne@comfy.org>
Co-authored-by: Subagent 5 <subagent@example.com>
Co-authored-by: Amp <amp@ampcode.com>

src/components/load3d/Load3DScene.vue

@@ -3,7 +3,8 @@
     ref="container"
     class="relative h-full w-full min-h-[200px]"
     data-capture-wheel="true"
-    @pointerdown.stop
+    tabindex="-1"
+    @pointerdown.stop="focusContainer"
     @pointermove.stop
     @pointerup.stop
     @mousedown.stop
@@ -45,6 +46,10 @@ const props = defineProps<{
 
 const container = ref<HTMLElement | null>(null)
 
+function focusContainer() {
+  container.value?.focus()
+}
+
 const { isDragging, dragMessage, handleDragOver, handleDragLeave, handleDrop } =
   useLoad3dDrag({
     onModelDrop: async (file) => {