refactor: rename the theme brand and every identifier from AKSRT to Jio

Full-project rename. `git grep -i aksrt` is now empty apart from the
DBMigrator 3.3.0 changelog note that documents the rename itself.

Code identifiers
- PHP namespace AKSRT\ -> Jio\ (composer PSR-4 + 378 declarations)
- global constants AKSRT_* -> JIO_* (VERSION/DIR/URI, PAY_TYPE_*, CAPTCHA_*)
- procedural functions, hooks, options, meta keys, table names,
  cookies, query vars, ajax actions and nonces: aksrt_ -> jio_
- text domain aksrt -> jio; Theme Name -> Jio
- JS globals: aksrtData/aksrtAdminData/... -> jioData/jioAdminData/...,
  window.AKSRTCaptcha & friends -> window.JioCaptcha, Aksrt* -> Jio*
- Vue components Aks*.vue -> Jio*.vue (15 files renamed + imports)
- CSS classes/data-attrs/custom properties: aksrt- -> jio-
- REST namespace aksrt/v1 -> jio/v1 (incl. payment gateway callbacks)

Storage layer
- DB tables {prefix}aksrt_* -> {prefix}jio_*; SCHEMA_VERSION 3.2.0 -> 3.3.0
  so dbDelta creates the new names on install
- widget id_base aksrt_widget_* -> jio_widget_*
- uninstall.php drop list rebuilt from the actual migrator tables
  (dropped 3 stale entries, added the 17 missing ones)

Removed
- hardcoded https://api.aksrt.com/version.json default update endpoint;
  get_endpoint() now reads the 「更新端点」 setting and do_check() no-ops
  when it is empty

Files renamed: inc/helpers/aksrt-*.php -> jio-*.php (54),
assets/admin/src/components/form/Aks*.vue -> Jio*.vue (15).
Build artifacts regenerated: assets/css/build, assets/js/runtime,
assets/admin/build.

Verified: audit-psr4-paths errors=0, audit-admin-contracts errors=0
(warnings=56, unchanged baseline), 813/813 widget-system tests,
44/44 phpunit, php -l clean across all PHP.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

inc/helpers/jio-null.php

@@ -0,0 +1,78 @@
+<?php
+/**
+ * Illustrated empty / null state helpers (legacy parity).
+ *
+ * @package Jio
+ */
+
+defined('ABSPATH') || exit;
+
+if (!function_exists('jio_null_image_url')) {
+	/**
+	 * Prefer theme null illustration; fall back to placeholder.
+	 */
+	function jio_null_image_url(string $file = 'null.svg'): string {
+		$file = sanitize_file_name($file);
+		if ($file === '') {
+			$file = 'null.svg';
+		}
+		$candidates = [
+			'/assets/img/' . $file,
+			'/img/' . $file,
+			'/assets/img/placeholder.svg',
+		];
+		foreach ($candidates as $rel) {
+			$path = get_template_directory() . $rel;
+			if (is_readable($path)) {
+				return get_template_directory_uri() . $rel;
+			}
+		}
+		return get_template_directory_uri() . '/assets/img/placeholder.svg';
+	}
+}
+
+if (!function_exists('jio_get_null')) {
+	/**
+	 * legacy-like empty illustration block.
+	 *
+	 * @param string|null $text    Caption (null → 暂无内容).
+	 * @param int|string  $margin  Vertical padding / top margin.
+	 * @param string      $img     Image file under assets/img.
+	 * @param string      $class   Extra class on wrapper.
+	 * @param int         $width   Image max width.
+	 * @param int         $height  Optional fixed height.
+	 */
+	function jio_get_null($text = null, $margin = 60, string $img = 'null.svg', string $class = '', int $width = 280, int $height = 0): string {
+		if (null === $text) {
+			$text = __('暂无内容', 'jio');
+		}
+		$margin = is_numeric($margin) ? (string) $margin : '60';
+		$caption = $text !== ''
+			? '<p style="margin-top:' . esc_attr($margin) . 'px;" class="em09 muted-3-color separator">' . esc_html((string) $text) . '</p>'
+			: '';
+		if ($height > 0) {
+			$style = $width > 0 ? 'max-width:' . (int) $width . 'px;' : '';
+			$style .= 'height:' . (int) $height . 'px;';
+		} else {
+			$style = $width > 0 ? 'width:' . (int) $width . 'px;' : '';
+		}
+		$style .= 'opacity:.7;';
+		$src = jio_null_image_url($img);
+		return '<div class="text-center jio-null ' . esc_attr($class) . '" style="padding:' . esc_attr($margin) . 'px 0;">'
+			. '<img class="no-imgbox jio-null__img" style="' . esc_attr($style) . '" src="' . esc_url($src) . '" alt="" decoding="async">'
+			. $caption
+			. '</div>';
+	}
+}
+
+if (!function_exists('jio_get_widget_null')) {
+	/**
+	 * Compact null for sidebar widgets.
+	 */
+	function jio_get_widget_null(string $text = '', int $margin = 24, int $width = 140): string {
+		if ($text === '') {
+			$text = __('暂无内容', 'jio');
+		}
+		return jio_get_null($text, $margin, 'null.svg', 'widget-empty jio-null--widget', $width);
+	}
+}