refactor: replace react markdown with streamdown (#32971)

Co-authored-by: Stephen Zhou <hi@hyoban.cc>
Co-authored-by: Stephen Zhou <38493346+hyoban@users.noreply.github.com>
Co-authored-by: yyh <92089059+lyzno1@users.noreply.github.com>

web/app/components/base/markdown-blocks/pre-code.tsx

@@ -1,23 +0,0 @@
-/**
- * @fileoverview PreCode component for rendering <pre> tags in Markdown.
- * Extracted from the main markdown renderer for modularity.
- * This is a simple wrapper around the HTML <pre> element.
- */
-import * as React from 'react'
-import { useRef } from 'react'
-
-function PreCode(props: { children: any }) {
-  const ref = useRef<HTMLPreElement>(null)
-
-  return (
-    <pre ref={ref}>
-      <span
-        className="copy-code-button"
-      >
-      </span>
-      {props.children}
-    </pre>
-  )
-}
-
-export default PreCode