fix: allow drag-to-pan over diagram nodes and compounds

Previously onPointerDown bailed out when the target was inside a node
(data-node-id), blocking pan entirely over nodes and compound groups.
Now panning always starts, and a didPan ref distinguishes drag from
click — node click handlers skip selection when the user was dragging.

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

ui/src/components/ProcessDiagram/ProcessDiagram.tsx

@@ -152,8 +152,12 @@ export function ProcessDiagram({
   );
 
   const handleNodeClick = useCallback(
-    (nodeId: string) => { onNodeSelect?.(nodeId); },
-    [onNodeSelect],
+    (nodeId: string) => {
+      // Suppress click if the pointer gesture was a drag (pan)
+      if (zoom.didPan.current) return;
+      onNodeSelect?.(nodeId);
+    },
+    [onNodeSelect, zoom.didPan],
   );
 
   const handleNodeDoubleClick = useCallback(