fix: code copy button now copies full code block

[olga-venger]

Closes #546

Problem

The copy button in code blocks only copies partial content. extractTextFromChildren recursively walks the React element tree using isValidElement, but next-mdx-remote/rsc renders code blocks as React Server Components, where lines are streamed to the client as lazy chunks. isValidElement returns false for these, so the function silently drops everything from that point in the tree.

Fix

Replace extractTextFromChildren with ref.current.textContent on the <pre> element. By the time the copy button is clicked, the browser has already rendered the full RSC output into real DOM text nodes and textContent reads from there directly, regardless of how React internally represented the element tree.
This also removes the dependency on isValidElement, which is a legacy React API not designed for RSC.