refactor: convert project from JavaScript to TypeScript and switch to pnpm

- Convert all .js/.jsx files to .ts/.tsx
- Add TypeScript configuration (tsconfig.json, tsconfig.node.json)
- Update vite.config.js to vite.config.ts
- Add TypeScript type definitions (@types/react, @types/react-dom, @types/node)
- Replace npm with pnpm (pnpm-lock.yaml)
- Add model configuration option for LLM API
- Update package.json scripts for TypeScript build
- All 52 tests passing

vite.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: './src/tests/setup.ts',
+  },
+})