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/Icons/IconController.php

@@ -0,0 +1,37 @@
+<?php
+/**
+ * @package AKSRT
+ */
+
+namespace AKSRT\Icons;
+
+use AKSRT\Admin\ControllerBase;
+
+defined('ABSPATH') || exit;
+
+/**
+ * REST endpoint serving the icon picker catalog.
+ */
+class IconController extends ControllerBase {
+
+	private static $instance = null;
+
+	public static function get_instance(): self {
+		if (null === self::$instance) self::$instance = new self();
+		return self::$instance;
+	}
+
+	public function register_routes(): void {
+		register_rest_route(self::API_NAMESPACE, '/icons/catalog', [
+			'methods'             => 'GET',
+			'callback'            => [$this, 'get_catalog'],
+			'permission_callback' => [$this, 'check_permission'],
+		]);
+	}
+
+	public function get_catalog(): \WP_REST_Response {
+		return $this->success([
+			'groups' => IconCatalog::groups(),
+		]);
+	}
+}