test(frontend): import vitest to stabilize typings

frontend/src/components/activity/ActivityFeed.test.tsx

@@ -1,4 +1,6 @@
 import { render, screen } from "@testing-library/react";
+import { describe, expect, it } from "vitest";
+
 
 import { ActivityFeed } from "./ActivityFeed";
 

frontend/src/lib/backoff.test.ts

@@ -1,4 +1,6 @@
 import { createExponentialBackoff } from "./backoff";
+import { describe, expect, it, vi } from "vitest";
+
 
 describe("createExponentialBackoff", () => {
   it("uses default options", () => {