Site Cloner: Load Customizer assets on block themes

[wputasic]

The "Clone Another WordCamp" screen renders as a dead shell whenever the active theme is a block theme: the section appears in the Customizer, but the site list never loads and search/filtering does nothing — no PHP or JS error anywhere. Since new WordCamp sites default to twentytwentyfour, every new organizer hits this. These are breakages 1 and 3 in #1315.

Root cause: the plugin registers its Customizer JS/CSS and the _wcscSettings localization (REST URL + nonce) only on admin_enqueue_scripts. customize.php never fires that hook itself — on classic themes it fires indirectly, because the Customizer's widgets component calls do_action( 'admin_enqueue_scripts', 'widgets.php' ). Since WordPress 5.9, the widgets component is not loaded when a block theme is active, so that indirect path disappears, register_scripts() never runs, and Site_Control::enqueue() silently no-ops on unregistered handles.

Fix: register the assets on customize_controls_enqueue_scripts — the hook customize.php always fires — at priority 1 so registration precedes WP_Customize_Manager::enqueue_control_scripts(). The assets are only consumed inside the Customizer, so registering them on every admin page was unnecessary anyway.

This restores cloning any classic-theme site regardless of the active theme. It deliberately does not address breakage 2 in #1315 (block-theme sites as clone sources) — that needs a larger rework, since a block-theme site's design lives in global styles/templates that the cloner's theme-switch + Additional-CSS payload doesn't capture.

See #1315.

Props dd32 (report & triage).

Screenshots

Before: the empty "Clone Another WordCamp" section shown in #1315. After the change, the site list, search, and filters render as they do on classic themes.

How to test the changes in this Pull Request:

1. On a WordCamp site, activate a block theme (e.g. Twenty Twenty-Five).
2. Go to Appearance → Clone Another WordCamp. Before this change the section is empty — no site list, no search. With it, the list loads and search/filters work.
3. Pick a site using a classic theme — the Customizer reloads with ?theme=<slug>, previews it, and Publish performs the clone.
4. Switch back to a classic theme and repeat, to confirm no regression.