phsl / new-api

- Replace Semi UI's MarkdownRender with react-markdown library for better performance and features
- Add comprehensive markdown rendering support including:
  * Math formulas with KaTeX
  * Code syntax highlighting with rehype-highlight
  * Mermaid diagrams support
  * GitHub Flavored Markdown (tables, task lists, etc.)
  * HTML preview for code blocks
  * Media file support (audio/video)
- Create new MarkdownRenderer component with enhanced features:
  * Copy code button with hover effects
  * Code folding for long code blocks
  * Responsive design for mobile devices
- Add white text styling for user messages to improve readability on blue backgrounds
- Update all components using markdown rendering:
  * MessageContent.js - playground chat messages
  * About/index.js - about page content
  * Home/index.js - home page content
  * NoticeModal.js - system notice modal
  * OtherSetting.js - settings page
- Install new dependencies: react-markdown, remark-math, remark-breaks, remark-gfm,
  rehype-katex, rehype-highlight, katex, mermaid, use-debounce, clsx
- Add comprehensive CSS styles in markdown.css for better theming and user experience
- Remove unused imports and optimize component imports

Breaking changes: None - maintains backward compatibility with existing markdown content