Fix dropdown race: use mousedown for outside-click handler

The document click handler registered on open could catch the same
click event that opened the dropdown, immediately closing it.
Using mousedown avoids the race since it fires before click.

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

apps/planner/app/components/ExportButton.tsx

@@ -47,14 +47,14 @@ export function ExportButton({ yjs }: { yjs: YjsState }) {
   const [open, setOpen] = useState(false);
   const ref = useRef<HTMLDivElement>(null);
 
-  // Close dropdown on outside click
+  // Close dropdown on outside mousedown
   useEffect(() => {
     if (!open) return;
     const handler = (e: MouseEvent) => {
       if (ref.current && !ref.current.contains(e.target as Node)) setOpen(false);
     };
-    document.addEventListener("click", handler);
-    return () => document.removeEventListener("click", handler);
+    document.addEventListener("mousedown", handler);
+    return () => document.removeEventListener("mousedown", handler);
   }, [open]);
 
   const handleExportRoute = useCallback(() => {