Add async-tasks UIWorker example

A UIWorker with a custom reply tool fans research out to three BaseWorker peers
via start_user_job_group; their progress streams to the client as ui-task cards
and the user can cancel a group mid-flight.

examples/multi-worker/ui-worker/async-tasks/client/vite.config.js

@@ -0,0 +1,7 @@
+import { defineConfig } from "vite";
+
+export default defineConfig({
+  server: {
+    port: 5173,
+  },
+});