feature: Code Block Optimization

On the client side, optimization of code block text is desired:

When copying Do not copy the ``` pairs at the beginning and end. Or show a copy button in the top right corner to copy the internal text of the code block like github.

Also would like to support syntax highlighting. (Just to leave a note, I know it's a bit of a pain to implement, it's not an urgent need, it would just look relatively comfortable)

source：

在客户端上，希望对代码块文本进行优化：

复制时 不复制开头结尾的```对。或者像github这样右上角显示复制按钮来复制代码块内部文本。

另外希望支持语法高亮。(只是想留个记录，我知道实现起来有点麻烦，并不是急迫的需求，只是看起来会相对舒服一点)

Start by locating the React Native client code that renders message code blocks and handles copying; no specific files or tests are named in the issue. Compare the current behavior with GitHub, then confirm whether the target is fence-free copying, a copy button, syntax highlighting, or a scoped subset. Done should be demonstrated by the agreed code-block behavior on the client.