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/base/pointerUtils.ts

@@ -0,0 +1,22 @@
+/**
+ * Utilities for pointer event handling
+ */
+
+/**
+ * Checks if a pointer or mouse event is a middle button input
+ * @param event - The pointer or mouse event to check
+ * @returns true if the event is from the middle button/wheel
+ */
+export function isMiddlePointerInput(
+  event: PointerEvent | MouseEvent
+): boolean {
+  if ('button' in event && event.button === 1) {
+    return true
+  }
+
+  if ('buttons' in event && typeof event.buttons === 'number') {
+    return event.buttons === 4
+  }
+
+  return false
+}