Add copy button on hover for code blocks

[mickdarling]

Summary

Add a copy-to-clipboard button that appears when hovering over code blocks, making it easy to copy code snippets.

Description

Currently, users must manually select and copy code from code blocks. A hover-activated copy button would improve UX significantly.

Proposed Behavior

• Copy button appears in the top-right corner of code blocks on mouse hover
• Clicking the button copies the code block content to clipboard
• Visual feedback on successful copy (e.g., checkmark icon or "Copied!" text)
• Button fades out when mouse leaves the code block area

Acceptance Criteria

• Copy button appears on hover for all code blocks (fenced and inline)
• Button copies raw code (without syntax highlighting markup)
• Visual feedback confirms successful copy
• Works across all preview themes
• Accessible via keyboard navigation