Implement voice assignment component (Task 10)

- Created VoiceAssignmentSection component with role-to-voice mapping
- Added language selector with American English, British English, French, Japanese
- Added speed slider (0.5x - 1.5x range)
- Implemented Generate Audiobook button with proper disabled state
- Integrated component into App.tsx with state management
- Added role detection API call on text submission
- Validates all roles have assigned voices before enabling generation

Requirements: 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 4.1, 4.2, 4.3, 4.4, 5.1, 5.2, 5.3

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

@@ -152,7 +152,7 @@
   - Mock fetch calls and verify request format
   - _Requirements: 9.6_
 
-- [ ] 9. Implement text input component
+- [x] 9. Implement text input component
   - Create TextInputSection component
   - Add large multiline textarea
   - Add "Next: Assign voices" button
@@ -172,7 +172,7 @@
   - Test validation message appears for empty text
   - _Requirements: 1.1, 1.3, 1.4_
 
-- [ ] 10. Implement voice assignment component
+- [x] 10. Implement voice assignment component
   - Create VoiceAssignmentSection component
   - Display "Assign voices to roles" title
   - Render role list with labels and dropdowns