test(frontend): replace jest with vitest

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/vitest.setup.ts

@@ -0,0 +1,15 @@
+/*
+ * SPDX-FileCopyrightText: 2025 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { expect, afterEach } from 'vitest'
+import { cleanup } from '@testing-library/react'
+import * as matchers from '@testing-library/jest-dom/matchers'
+
+expect.extend(matchers)
+
+afterEach(() => {
+  cleanup()
+})