Add click-to-copy for fenced code blocks

[bganesh]

Clicking any fenced code block in the preview now copies its content to the clipboard.

Changes:

• Each <pre><code> block is decorated with a code-block--copyable class, a role="button", and keyboard support (Enter/Space) for accessibility
• Labels "Click to copy" / "Copied" / "Copy failed" indicate status on hover / click
• Outline appears on hover/focus to indicate interactivity
• Selection-aware: clicking while text is selected inside the block does nothing, so normal text selection still works
• The copy label is hidden on print (@media print)

Ensured CSS won't clash if other PRs are accepted. .document pre.code-block--copyable::after can be merged with with previous entry for display: none;

Fixes #9