perf: replace useViewport with narrow useStore selector in GroupControlsOverlay

useViewport() re-renders on every pan AND zoom. The narrow selector
useStore((s) => s.transform[2]) only fires when zoom changes, eliminating
unnecessary group overlay re-renders during panning.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/GroupsOverlay.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useCallback, useState, useRef, useEffect } from "react";
-import { useViewport, ViewportPortal } from "@xyflow/react";
+import { useStore, ViewportPortal } from "@xyflow/react";
 import { useWorkflowStore, GROUP_COLORS } from "@/store/workflowStore";
 import { GroupColor } from "@/types";
 
@@ -492,7 +492,7 @@ export function GroupBackgroundsPortal() {
 // Renders group controls (headers, resize handles) using ViewportPortal above nodes
 export function GroupControlsOverlay() {
   const { groups } = useWorkflowStore();
-  const { zoom } = useViewport();
+  const zoom = useStore((s) => s.transform[2]);
 
   const groupIds = Object.keys(groups);