Migrate from React Markdown to Streamdown

[haydenbleasel]

Howdy!

This PR migrates example apps from react-markdown (plus plugins) to Streamdown, a modern markdown renderer purpose-built for AI streaming applications we're building at Vercel. We're using it on a few projects like ChatSDK and thought you might enjoy it too.

Benefits

• Built for AI streaming - Handles incomplete markdown gracefully during real-time content generation
• Fewer dependencies - Replaces 5 packages (react-markdown, rehype-highlight, rehype-raw, rehype-sanitize, remark-gfm) with 1
• Better out-of-box experience - Built-in Tailwind typography, syntax highlighting, and GFM support
• Enhanced security - Native protection against malicious markdown and prompt injection
• CJK language support - Proper emphasis rendering for Chinese, Japanese, and Korean
• Performance optimized - Lots of effort gone in to memoization, LRU caches, etc. to make the fastest and most performant markdown streaming experience.
• Interactive code blocks and tables - Users can download and copy tables / code block data
• Mermaid support - built in support for Mermaid diagrams

Also, while I was here I fixed a handful of dependency issues (many unused, one missing).