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}