fix: use non-passive wheel listener to prevent page scroll during diagram zoom

React's onWheel is passive by default, so preventDefault() doesn't stop
page scrolling. Attach native wheel listener with { passive: false } via
useEffect instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/ProcessDiagram.tsx

@@ -193,8 +193,8 @@ export function ProcessDiagram({
       )}
 
       <svg
+        ref={zoom.svgRef}
         className={styles.svg}
-        onWheel={zoom.onWheel}
         onPointerDown={zoom.onPointerDown}
         onPointerMove={zoom.onPointerMove}
         onPointerUp={zoom.onPointerUp}

ui/src/components/ProcessDiagram/useZoomPan.ts

@@ -1,4 +1,4 @@
-import { useCallback, useRef, useState } from 'react';
+import { useCallback, useEffect, useRef, useState } from 'react';
 
 interface ZoomPanState {
   scale: number;
@@ -20,19 +20,24 @@ export function useZoomPan() {
   const isPanning = useRef(false);
   const panStart = useRef({ x: 0, y: 0 });
   const containerRef = useRef<HTMLDivElement>(null);
+  const svgRef = useRef<SVGSVGElement>(null);
 
   const clampScale = (s: number) => Math.min(MAX_SCALE, Math.max(MIN_SCALE, s));
 
   /** Returns the CSS transform string for the content <g> element. */
   const transform = `translate(${state.translateX}px, ${state.translateY}px) scale(${state.scale})`;
 
-  const onWheel = useCallback(
-    (e: React.WheelEvent<SVGSVGElement>) => {
+  // Attach wheel listener with { passive: false } so preventDefault() stops page scroll.
+  // React's onWheel is passive by default and cannot prevent scrolling.
+  useEffect(() => {
+    const svg = svgRef.current;
+    if (!svg) return;
+    const handler = (e: WheelEvent) => {
       e.preventDefault();
       const direction = e.deltaY < 0 ? 1 : -1;
       const factor = 1 + direction * ZOOM_STEP;
 
-      const rect = (e.currentTarget as SVGSVGElement).getBoundingClientRect();
+      const rect = svg.getBoundingClientRect();
       const cursorX = e.clientX - rect.left;
       const cursorY = e.clientY - rect.top;
 
@@ -45,9 +50,10 @@ export function useZoomPan() {
           translateY: cursorY - scaleRatio * (cursorY - prev.translateY),
         };
       });
-    },
-    [],
-  );
+    };
+    svg.addEventListener('wheel', handler, { passive: false });
+    return () => svg.removeEventListener('wheel', handler);
+  }, []);
 
   const onPointerDown = useCallback(
     (e: React.PointerEvent<SVGSVGElement>) => {
@@ -158,10 +164,10 @@ export function useZoomPan() {
   return {
     state,
     containerRef,
+    svgRef,
     transform,
     panTo,
     resetView,
-    onWheel,
     onPointerDown,
     onPointerMove,
     onPointerUp,