test: import jest-dom matchers in SourceSelector test

SourceSelector.test.tsx uses toBeInTheDocument but never imported
@testing-library/jest-dom, and vitest.config.ts has no setupFiles to
register the matchers globally. It only passed when test ordering
happened to leak the matcher from another file, making CI intermittently
red on main (e.g. the #652 merge). Importing jest-dom directly makes the
test self-sufficient and deterministic.

src/components/launch/SourceSelector.test.tsx

@@ -1,3 +1,4 @@
+import "@testing-library/jest-dom";
 import { fireEvent, render, screen, waitFor } from "@testing-library/react";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import { SourceSelector } from "./SourceSelector";