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/components/TextInputSection.tsx

@@ -0,0 +1,71 @@
+import { useState } from 'react';
+
+interface TextInputSectionProps {
+  text: string;
+  onChange: (text: string) => void;
+  onNext: () => void;
+  isLoading: boolean;
+  disabled: boolean;
+}
+
+export default function TextInputSection({
+  text,
+  onChange,
+  onNext,
+  isLoading,
+  disabled
+}: TextInputSectionProps) {
+  const [showValidation, setShowValidation] = useState(false);
+
+  const handleNext = () => {
+    if (text.trim() === '') {
+      setShowValidation(true);
+      return;
+    }
+    setShowValidation(false);
+    onNext();
+  };
+
+  const handleTextChange = (e: React.ChangeEvent<HTMLTextAreaElement>) => {
+    onChange(e.target.value);
+    if (e.target.value.trim() !== '') {
+      setShowValidation(false);
+    }
+  };
+
+  const isButtonDisabled = disabled || isLoading || text.trim() === '';
+
+  return (
+    <div className="w-full max-w-4xl mx-auto p-6">
+      <h2 className="text-2xl font-bold mb-4">Enter Your Text</h2>
+      
+      <div className="mb-4">
+        <textarea
+          value={text}
+          onChange={handleTextChange}
+          placeholder="Enter your text here. Use patterns like 'Name:' or '[Name]' to mark different speakers..."
+          className="w-full h-96 p-4 border-2 border-gray-300 rounded-lg focus:border-blue-500 focus:outline-none resize-y text-base"
+          disabled={disabled || isLoading}
+        />
+      </div>
+
+      {showValidation && text.trim() === '' && (
+        <div className="mb-4 p-3 bg-red-100 border border-red-400 text-red-700 rounded">
+          Please enter some text before proceeding.
+        </div>
+      )}
+
+      <button
+        onClick={handleNext}
+        disabled={isButtonDisabled}
+        className={`px-6 py-3 rounded-lg font-semibold text-white transition-colors ${
+          isButtonDisabled
+            ? 'bg-gray-400 cursor-not-allowed'
+            : 'bg-blue-600 hover:bg-blue-700 cursor-pointer'
+        }`}
+      >
+        {isLoading ? 'Processing...' : 'Next: Assign voices'}
+      </button>
+    </div>
+  );
+}