Move from React Markdown to Streamdown

[cantemizyurek]

Instead of using React Markdown, Stream Down offers better support for LLM outputs and includes built-in syntax highlighting powered by Shiki.

[cantemizyurek]

@mattpocock If you want I can make the swap this would also solve #97 and #101

[mattpocock]

Yeah seems like a no-brainer, love it.

PR to #240?