Implement text input component (Task 9)

- Create TextInputSection component with large multiline textarea
- Add 'Next: Assign voices' button
- Implement text validation (non-empty)
- Show validation message for empty text
- Disable button when text is empty
- Update App.tsx to integrate TextInputSection
- Requirements: 1.1, 1.2, 1.3, 1.4

frontend/src/App.tsx

@@ -1,34 +1,50 @@
 import { useState } from 'react'
-import reactLogo from './assets/react.svg'
-import viteLogo from '/vite.svg'
 import './App.css'
+import TextInputSection from './components/TextInputSection'
 
 function App() {
-  const [count, setCount] = useState(0)
+  const [text, setText] = useState('')
+  const [phase, setPhase] = useState<'input' | 'assignment' | 'generated'>('input')
+  const [isLoading] = useState(false)
+
+  const handleTextChange = (newText: string) => {
+    setText(newText)
+  }
+
+  const handleNext = () => {
+    // TODO: Implement role detection and phase transition
+    console.log('Moving to voice assignment phase with text:', text)
+    setPhase('assignment')
+  }
 
   return (
-    <>
-      <div>
-        <a href="https://vite.dev" target="_blank">
-          <img src={viteLogo} className="logo" alt="Vite logo" />
-        </a>
-        <a href="https://react.dev" target="_blank">
-          <img src={reactLogo} className="logo react" alt="React logo" />
-        </a>
+    <div className="min-h-screen bg-gray-50">
+      <div className="container mx-auto py-8">
+        <h1 className="text-4xl font-bold text-center mb-8">Text to Audiobook</h1>
+        
+        {phase === 'input' && (
+          <TextInputSection
+            text={text}
+            onChange={handleTextChange}
+            onNext={handleNext}
+            isLoading={isLoading}
+            disabled={false}
+          />
+        )}
+
+        {phase === 'assignment' && (
+          <div className="text-center">
+            <p>Voice assignment phase (to be implemented)</p>
+          </div>
+        )}
+
+        {phase === 'generated' && (
+          <div className="text-center">
+            <p>Audio player phase (to be implemented)</p>
+          </div>
+        )}
       </div>
-      <h1>Vite + React</h1>
-      <div className="card">
-        <button onClick={() => setCount((count) => count + 1)}>
-          count is {count}
-        </button>
-        <p>
-          Edit <code>src/App.tsx</code> and save to test HMR
-        </p>
-      </div>
-      <p className="read-the-docs">
-        Click on the Vite and React logos to learn more
-      </p>
-    </>
+    </div>
   )
 }