fix mmb navigation when click starts on Vue nodes (#5921)

## Summary

Fixes https://github.com/Comfy-Org/ComfyUI_frontend/issues/5860
by updating Vue node pointer interactions to forward middle mouse button
events to canvas instead of handling them locally.

## Review Focus

Middle mouse button event detection logic using both `button` property
and `buttons` bitmask for cross-browser compatibility. Test coverage for
pointer event forwarding behavior.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5921-fix-mmb-navigation-when-click-starts-on-Vue-nodes-2826d73d3650819688eec4600666755d)
by [Unito](https://www.unito.io)

---------

Co-authored-by: DrJKL <DrJKL@users.noreply.github.com>

src/renderer/core/canvas/useCanvasInteractions.ts

@@ -1,5 +1,6 @@
 import { computed } from 'vue'
 
+import { isMiddlePointerInput } from '@/base/pointerUtils'
 import { useSettingStore } from '@/platform/settings/settingStore'
 import { useCanvasStore } from '@/renderer/core/canvas/canvasStore'
 import { app } from '@/scripts/app'
@@ -59,6 +60,11 @@ export function useCanvasInteractions() {
    * be forwarded to canvas (e.g., space+drag for panning)
    */
   const handlePointer = (event: PointerEvent) => {
+    if (isMiddlePointerInput(event)) {
+      forwardEventToCanvas(event)
+      return
+    }
+
     // Check if canvas exists using established pattern
     const canvas = getCanvas()
     if (!canvas) return