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

[TeigenZhang]

Adds a one-click copy button to every fenced code block in the response viewer.

• Each code block is wrapped in a positioned .rv-code-wrap toolbar pinned top-right, outside the <pre> scroll container so the 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.

Test plan

• esbuild parses app.js / styles.css cleanly
• Verified in the UI: copy button appears on code blocks, copies the raw source, shows ✓ on success (both HTTPS and plain HTTP)

🤖 Generated with Claude Code

[Ark0N]

Thank you very much for this — really clean, well-scoped contribution! 🙏

I reviewed it end to end and everything checks out:

• The DOM restructure preserves the existing diagram line-wrap toggle: diagrams keep the rv-diagram-wrap class, so the delegated handler's closest('.rv-diagram-wrap') / querySelector('pre.rv-diagram') still resolve.
• CSS specificity is correct — the new .rv-code-wrap.rv-diagram-wrap > pre.rv-diagram rule properly widens the padding reserve to 76px for the two-button case, and .rv-code-actions .rv-wrap-toggle { position: static } neatly re-flows the toggle into the flex toolbar.
• The execCommand fallback in _copyText() is a nice touch — copy works over plain HTTP (tunnel) too.
• Reusing the ⎘ glyph keeps it visually consistent with the file-preview copy button.

CI is green (Typecheck & Lint). Merging now. Thanks again! 🎉