feat: Refactor ItineraryView and StepList components for improved functionality and styling

- Updated ItineraryView to manage steps directly and handle itinerary updates through props.
- Introduced StepList component to encapsulate step management logic and UI.
- Enhanced theme styling with a new CSS file for a more visually appealing design.
- Adjusted default theme settings to use the new standard theme.
- Modified page routes to accommodate the new component structure and ensure proper data flow.

Author: soranjiro