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/package.json

@@ -18,7 +18,8 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run"
   },
   "dependencies": {
     "@modelcontextprotocol/sdk": "^1.0.3",
@@ -40,6 +41,8 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.11.1",
+    "@testing-library/jest-dom": "^6.6.3",
+    "@testing-library/react": "^16.2.0",
     "@types/node": "^22.7.5",
     "@types/react": "^18.3.10",
     "@types/react-dom": "^18.3.0",
@@ -50,10 +53,12 @@
     "eslint-plugin-react-hooks": "^5.1.0-rc.0",
     "eslint-plugin-react-refresh": "^0.4.12",
     "globals": "^15.9.0",
+    "jsdom": "^26.0.0",
     "postcss": "^8.4.47",
     "tailwindcss": "^3.4.13",
     "typescript": "^5.5.3",
     "typescript-eslint": "^8.7.0",
-    "vite": "^5.4.8"
+    "vite": "^5.4.8",
+    "vitest": "^3.0.0"
   }
 }