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/Core/ScriptsPart1b.php

@@ -25,17 +25,12 @@ trait ScriptsPart1b {
 			[],
 			'3.4.1'
 		);
-		wp_enqueue_style(
-			'fontawesome5',
-			$base_uri . '/assets/fontawesome5/css/all.min.css',
-			['bootstrap'],
-			'5.15.4'
-		);
+		aksrt_enqueue_fontawesome(['bootstrap']);
 		$main_css_path = get_template_directory() . '/assets/css/main.min.css';
 		wp_enqueue_style(
 			'aksrt-main',
 			$base_uri . '/assets/css/main.min.css',
-			['fontawesome5'],
+			['aksrt-font-awesome'],
 			file_exists($main_css_path) ? (string) filemtime($main_css_path) : $version
 		);
         wp_enqueue_style(