Add copy controls to docs code blocks

[Copilot]

The docs viewer renders workshop markdown without any way to quickly copy sample commands or code. This adds per-block copy controls with lightweight feedback so readers can grab snippets directly from the rendered guide.

• Code block actions

  • Injects a copy button into each rendered markdown pre > code block after marked.parse()
  • Copies the block’s text content, not HTML, so shell commands and code paste cleanly
  • Avoids duplicate controls when the viewer re-initializes

• Clipboard handling

  • Uses the Clipboard API when available
  • Falls back to an off-screen textarea + execCommand('copy') path for older or restricted contexts
  • Keeps the fallback encapsulated in the docs viewer script

• User feedback + styling

  • Adds inline state feedback: Copy → Copied! / Failed
  • Styles the control to match the existing docs theme
  • Reserves space in the code block header so the button does not overlap content

<pre>
  <code>uv run pytest</code>
  <button class="code-copy-btn">Copy</button>
</pre>