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/tsconfig.test.json

@@ -0,0 +1,7 @@
+{
+  "extends": "./tsconfig.app.json",
+  "compilerOptions": {
+    "types": ["vitest/globals", "@testing-library/jest-dom"]
+  },
+  "include": ["src/**/*.test.tsx", "src/**/*.test.ts", "test/**/*.ts"]
+}