Port ACF's _acf-copyable.js for click-to-copy field keys on list tables

What

On the Field Groups / Post Types / Taxonomies / Options Pages list tables, the Key column (.column-acf-key) has no copy affordance in SCF. Upstream wraps the key in a .copyable span and handles click-to-copy from a dedicated assets/src/js/_acf-copyable.js, which SCF has never ported — SCF only has the field-group editor copyable (in _field-group-field.js), which works fine and is unrelated.

Why now

#506 synced the sass partials from ACF 6.8.5, so the list-table copyable CSS (_list-table.scss: hover copy icon, green check on .copied) already ships — it's inert until the JS creates the .copyable markup. Porting the JS completes the feature with zero further CSS work.

Where upstream has it

Free ACF repo, tag 6.8.5, assets/src/js/_acf-copyable.js: an acf.Model that gates on the relevant pagenow screens, wraps .column-acf-key contents, and handles the copy click (including the copy-unsupported fallback for non-secure contexts). Check how upstream enqueues it (bundle entry vs standalone) and mirror that in SCF's build.

Acceptance

Hovering a key in the list-table Key column shows the copy icon; clicking copies the key and swaps to the green check, matching the existing editor behavior.

Found while reviewing #506 (the "Copyable field keys" family of its CSS delta is currently inert).

🤖 Generated with Claude Code

Compare upstream ACF 6.8.5's assets/src/js/_acf-copyable.js with SCF's existing _field-group-field.js, then inspect how the script is enqueued in SCF's build. Confirm that the existing _list-table.scss styles apply to the generated markup. Done means list-table keys show the copy icon, copy successfully, and switch to the green check, including the fallback behavior described in the issue.