Add copy-to-clipboard button on code blocks in MarkdownBlock

[alimaandev]

Summary

desktop/src/components/chat/MarkdownBlock.tsx renders syntax-highlighted code blocks (highlight.js), but there is no per-code-block copy button. MessageBubble already has a copy button for whole messages (see useCopyButton in MessageBubble.tsx).

Goal

Add a small copy button in the top-right corner of each <pre>\ code block rendered by MarkdownBlock. Reuse the same clipboard fallback pattern used in MessageBubble.tsx (navigator.clipboard with execCommand fallback).

Tasks

• Add a copy button overlay on code blocks
• Show a transient 'Copied' state for ~1.5s
• Reuse the existing copy helper pattern — do not add a new util

Acceptance criteria

• Clicking copy on a code block copies only that block's code
• Button shows 'Copied' feedback then reverts
• Existing message copy behavior unchanged

Labels

good first issue, help wanted