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/vitest.config.ts

@@ -0,0 +1,20 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+import path from 'path'
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./test/setupTests.ts'],
+    typecheck: {
+      tsconfig: './tsconfig.test.json'
+    }
+  },
+  resolve: {
+    alias: {
+      '@': path.resolve(__dirname, './src'),
+    },
+  },
+})