fix: ensure proper DOM cleanup between tests in CI environment

Add explicit React Testing Library cleanup in test setup to prevent
DOM contamination between tests when running in CI mode with single
thread isolation. This resolves test failures in CI where multiple
VocabularyTable components would interfere with each other due to
shared element IDs and insufficient cleanup.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: jonphipps

packages/theme/src/tests/setup.ts

@@ -1 +1,8 @@
-import '@testing-library/jest-dom';
+import '@testing-library/jest-dom';
+import { cleanup } from '@testing-library/react';
+import { afterEach } from 'vitest';
+
+// Ensure DOM cleanup after each test
+afterEach(() => {
+  cleanup();
+});