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().

src/Admin/Admin.php

@@ -111,11 +111,16 @@ class Admin {
         }
 
         // Use the same local FA5 pack as the public stack.
-        wp_enqueue_style(
-            'aksrt-font-awesome',
-            get_template_directory_uri() . '/assets/fontawesome5/css/all.min.css',
-            [],
-            '5.15.4'
+        aksrt_enqueue_fontawesome();
+
+        // Frontend SVG sprite (defines window.aksrt_svgs and injects the symbol
+        // sheet) so schema icon fields can preview built-in zibsvg values.
+        wp_enqueue_script(
+            'aksrt-svg-sprite',
+            get_template_directory_uri() . '/assets/js/runtime/svg-icon.js',
+            ['jquery'],
+            (string) filemtime(get_template_directory() . '/assets/js/runtime/svg-icon.js'),
+            true
         );
 
         wp_enqueue_script(
@@ -186,6 +191,7 @@ class Admin {
             CommentController::class,
             MessageController::class,
             AIController::class,
+            \AKSRT\Icons\IconController::class,
         ];
 
         foreach ($controllers as $class_name) {