feat: Add responsive styling and mobile support

- Applied Tailwind CSS responsive classes across all components
- Implemented mobile-first design approach with breakpoints (sm, md, lg)
- Added touch-friendly interactions with min-h-[44px] touch targets
- Enhanced App.tsx with responsive padding, headings, and alerts
- Updated TextInputSection with responsive textarea and full-width mobile button
- Improved VoiceAssignmentSection with flexible layouts and mobile-optimized controls
- Enhanced AudioPlayerSection with responsive sizing and touch support
- Added custom CSS for range slider with larger thumb for mobile
- Improved select dropdowns with visible arrows and proper padding
- Added touch-manipulation class to prevent double-tap zoom
- Added active states for better mobile feedback
- Updated index.html with proper title and meta description
- Created RESPONSIVE_IMPROVEMENTS.md documentation

Requirements: 12.1, 12.2, 12.3, 12.4

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

@@ -235,7 +235,7 @@
   - Test download button triggers file save
   - _Requirements: 7.2, 7.3, 7.4_
 
-- [ ] 12. Implement main App component with workflow orchestration
+- [x] 12. Implement main App component with workflow orchestration
   - Create App component with state management
   - Implement phase transitions (input → assignment → generated)
   - Wire up text input to role detection
@@ -273,7 +273,7 @@
   - Test loading states appear during API calls
   - _Requirements: 8.1, 8.2, 8.4_
 
-- [ ] 13. Add responsive styling and mobile support
+- [x] 13. Add responsive styling and mobile support
   - Apply Tailwind CSS classes for responsive layout
   - Ensure text areas and controls are mobile-friendly
   - Test on different screen sizes