feat(icons): add the FA5 catalog, REST endpoint and unified icon helpers

Mirror the reference theme's universal picker on FA5: a generated full
catalog of the bundled FA5 Free 5.15.4 fonts (src/Icons/fa5-icons.php,
generated from the shipped CSS + SVG fonts), a zibsvg group parsed from
the runtime sprite so it cannot drift, and GET /aksrt/v1/icons/catalog.

Shared admin pieces: aksrt_icon_markup()/aksrt_sanitize_icon_value()
with an SVG allowlist, the picker field/modal assets, and one canonical
aksrt-font-awesome handle registered through aksrt_enqueue_fontawesome().

inc/core-hooks.php

@@ -448,12 +448,7 @@ add_filter('manage_post_posts_columns', function ($columns) {
 
 add_action('admin_enqueue_scripts', function ($hook) {
     if ($hook !== 'edit.php') return;
-    wp_enqueue_style(
-        'aksrt-font-awesome',
-        get_template_directory_uri() . '/assets/fontawesome5/css/all.min.css',
-        [],
-        '5.15.4'
-    );
+    aksrt_enqueue_fontawesome();
 });
 
 add_action('manage_post_posts_custom_column', function ($column, $post_id) {