fix: diagram zoom not working on initial render

The wheel event listener was attached in a useEffect with empty deps,
but the SVG element doesn't exist during the loading state. Switch
svgRef from a plain ref to a callback ref that triggers re-attachment
when the SVG element becomes available after data loads.

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

ui/src/components/ProcessDiagram/useZoomPan.ts

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useRef, useState } from 'react';
+import { useCallback, useEffect, useRef, useState, type RefCallback } from 'react';
 
 interface ZoomPanState {
   scale: number;
@@ -23,7 +23,13 @@ export function useZoomPan() {
   const didPan = useRef(false);
   const panStart = useRef({ x: 0, y: 0 });
   const containerRef = useRef<HTMLDivElement>(null);
-  const svgRef = useRef<SVGSVGElement>(null);
+  const svgElRef = useRef<SVGSVGElement | null>(null);
+  const [svgReady, setSvgReady] = useState(false);
+
+  const svgRef: RefCallback<SVGSVGElement> = useCallback((el: SVGSVGElement | null) => {
+    svgElRef.current = el;
+    setSvgReady(!!el);
+  }, []);
 
   const clampScale = (s: number) => Math.min(MAX_SCALE, Math.max(MIN_SCALE, s));
 
@@ -32,8 +38,9 @@ export function useZoomPan() {
 
   // Attach wheel listener with { passive: false } so preventDefault() stops page scroll.
   // React's onWheel is passive by default and cannot prevent scrolling.
+  // Re-runs when the SVG element becomes available (e.g. after loading state clears).
   useEffect(() => {
-    const svg = svgRef.current;
+    const svg = svgElRef.current;
     if (!svg) return;
     const handler = (e: WheelEvent) => {
       e.preventDefault();
@@ -56,7 +63,7 @@ export function useZoomPan() {
     };
     svg.addEventListener('wheel', handler, { passive: false });
     return () => svg.removeEventListener('wheel', handler);
-  }, []);
+  }, [svgReady]);
 
   const onPointerDown = useCallback(
     (e: React.PointerEvent<SVGSVGElement>) => {