Fix flaky CI frontend job: unmount React trees after each test

React 19's scheduler could flush deferred work after vitest tore down the
jsdom environment (window is not defined), failing the run on slow runners.
Register a global @testing-library/react cleanup in the vitest setup.

Author: vsdudakov

frontend/src/test-setup.ts

@@ -22,3 +22,12 @@ Object.defineProperty(window, "matchMedia", {
     dispatchEvent: () => false,
   }),
 });
+
+// Unmount rendered trees after every test so React's scheduler has no
+// deferred work left when vitest tears down the jsdom environment.
+import { cleanup } from "@testing-library/react";
+import { afterEach } from "vitest";
+
+afterEach(() => {
+  cleanup();
+});