feat(web): one-click copy button on response-viewer code blocks (#98)

Wrap every fenced code block in the response viewer with a positioned
.rv-code-wrap toolbar (outside the <pre> scroll container so buttons stay put
during horizontal scroll). All blocks get a copy button; ASCII diagrams keep
their existing line-wrap toggle alongside it.

_copyText() prefers the async Clipboard API and falls back to a hidden-textarea
+ execCommand path, so copy works over plain HTTP too. The button shows a 1.5s
✓ / ✕ feedback state after each attempt.

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>