Add frontend unit tests

- Set up Vitest with React Testing Library
- Add comprehensive tests for Button and ListPane components
- Configure TypeScript for test environment
- Add test type declarations

Co-Authored-By: ashwin@anthropic.com <ashwin@anthropic.com>

client/test/setupTests.ts

@@ -0,0 +1,6 @@
+/// <reference types="vitest/globals" />
+/// <reference types="@testing-library/jest-dom" />
+import '@testing-library/jest-dom/vitest'
+
+// Add any additional test setup, custom matchers, or global mocks here
+// This file runs before each test file