feat: implement script review workflow in App component

- Add 'review' phase to application state
- Add scriptSegments state variable for storing AI-generated segments
- Add isGeneratingScript loading state for script generation
- Implement handleGenerateScript() to call API and transition to review phase
- Implement handleSkipReview() to bypass review and go directly to generation
- Implement handleSegmentChange() to update speaker assignments in segments
- Update handleGenerate() to use edited segments when available
- Wire up ScriptReviewSection component in review phase
- Update phase transitions: input → assignment → review → generated
- Add loading indicator for script generation

Validates Requirements: 15.1, 15.3, 15.6, 15.7, 15.8
Task: 22. Update App component for script review workflow

.kiro/specs/text-to-audiobook/tasks.md

@@ -424,7 +424,7 @@
   - Test skip review button works
   - _Requirements: 15.3, 15.4, 15.5, 15.8_
 
-- [ ] 22. Update App component for script review workflow
+- [x] 22. Update App component for script review workflow
   - Add 'review' phase to application state
   - Add scriptSegments state variable
   - Add isGeneratingScript loading state