feat: scaffold Flask + React app with schema and JSON error handling

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

src/App.tsx

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+export function App() {
+  return <h1 className="p-6 text-2xl font-semibold">My Library</h1>
+}

src/index.css

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+@import "tailwindcss";

src/main.tsx

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+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+import { App } from './App'
+import './index.css'
+
+createRoot(document.getElementById('root')!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+)

src/test/setup.ts

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+import '@testing-library/jest-dom/vitest'
+import { cleanup } from '@testing-library/react'
+import { afterEach } from 'vitest'
+
+// Vitest globals are off, so Testing Library cannot register its own cleanup.
+afterEach(() => cleanup())