phsl / new-api

- Remove duplicate thinking content rendering logic from MessageContent component
- Import and utilize ThinkingContent component for consistent thinking display
- Clean up unused icon imports (ChevronRight, ChevronUp, Brain)
- Consolidate "思考中..." header text logic into single component
- Reduce code duplication by ~70 lines while maintaining all functionality
- Improve component separation of concerns and maintainability

The MessageContent component now delegates thinking content rendering to the
dedicated ThinkingContent component, eliminating the previously duplicated
UI logic and state management for thinking processes.