Restore code-block copy button

[drmowinckels]

Context

A copy-to-clipboard button for code blocks (assets/js/copy-code.js + matching CSS in assets/css/components/syntax.css + bundling in layouts/partials/footer/scripts.html) was prototyped during the rebrand but reverted before launch because the toast confirmation wasn't behaving correctly.

What we want

Code blocks (.highlight) should have a copy-to-clipboard button in the top-right corner with:

• Visible-on-hover affordance
• Click → write code.innerText to clipboard via navigator.clipboard.writeText
• Visual feedback on success (icon swap to check, brief toast "Copied!")
• Failure path (toast "Copy failed")
• Dark-mode parity using CSS variables (no hardcoded greens — the previous attempt hardcoded #0a5030 / #86efac for the success state)

Suggested approach

• Reuse the previous JS scaffold from git history (revert commit on `rebrand` branch removed it; check earlier WIP).
• Investigate why the toast didn't render — likely a positioning/stacking-context issue with `.highlight.has-copy-btn { position: relative }` and the toast appended to `block.parentNode`.
• Use a CSS variable (e.g. `--color-success` or similar) for the success state instead of hardcoded hex.
• Consider Alpine for the show/hide of the toast rather than imperative `setTimeout` chains.

Out of scope

• Keyboard-only invocation (separate accessibility ticket if needed).