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.app.json

@@ -4,6 +4,7 @@
     "paths": {
       "@/*": ["./src/*"]
     },
+    "types": ["vitest/globals", "@testing-library/jest-dom"],
 
     "target": "ES2020",
     "useDefineForClassFields": true,
@@ -26,5 +27,5 @@
     "noFallthroughCasesInSwitch": true,
     "resolveJsonModule": true
   },
-  "include": ["src"]
+  "include": ["src", "test"]
 }