feat: copy button on preview code blocks, and long lines wrap

[productengineered]

Code blocks in preview often hold something to paste somewhere else: a prompt, a shell command, a snippet. This adds a copy button to every code block in preview. Long lines now wrap, so a long prompt reads in full instead of sitting behind a horizontal scrollbar.

I know CONTRIBUTING says feature PRs get closed. This one is small and preview-only, and I use it every day for workshop handouts. Happy to adjust it or drop it.

What

• The button. Every code block in preview gets a copy button in its top-right corner, always shown. A click puts the block's code on the clipboard exactly (navigator.clipboard.writeText) and shows a check for --dur-copied, then the icon returns. The icons are inline SVG drawn with currentColor, so they follow every theme. New tokens: --code-copy-size, --code-copy-inset, --dur-copied.
• The button never covers text. The block's right padding reserves the button's column, and preview code wraps (white-space: pre-wrap; overflow-wrap: anywhere), so text stops before the button. export.css is unchanged, so HTML and PDF exports keep their current code styling.
• The <pre> stays untouched. The button sits in a .code-block wrapper beside the <pre>, not inside it. The block's text stays exactly the code (the double-click mapping reads it), and the button doesn't scroll with the content. Exports and Copy as Rich Text never include it.
• Interaction details. Clicking the button keeps focus on the preview pane (its mousedown is prevented), so Space and the arrow keys still scroll. A double-click on the button copies instead of dropping into the editor.
• Help: one sentence in the preview paragraph of help.md.

Tests

• Layer: end-to-end (ui/preview.ts).
• Case / test added or extended: scripts/e2e/cases/code-copy.mjs. It checks:
  • the button is shown without hovering,
  • no block scrolls horizontally, and the long prompt wraps,
  • every text rect ends left of the button,
  • the button adds no text to the block,
  • each block copies verbatim (the clipboard is stubbed so a run never overwrites the developer's own),
  • focus stays on the pane,
  • a double-click on the button stays in preview,
  • the check reverts.
• pnpm typecheck && pnpm test pass, pnpm e2e passes (7/7), and pnpm test:coverage stays above the floors.
• I also tested it with a real clipboard on a local build: clicking copied the prompt exactly, checked with pbpaste.