feat(branding): default the site icons to assets/img/favicon.png

- Point the 浏览器图标 / 移动端图标 schema defaults (PHP + VUE) at
  assets/img/favicon.png instead of an empty media field.
- Fall back to the same PNG in Settings::favicon_url() when no custom
  favicon and no native site_icon are configured.
- Add Settings::mobile_icon_url() and emit apple-touch-icon from SEOHead;
  the 移动端图标 field previously had no consumer and never reached the page.

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

src/Core/SEOHead.php

@@ -236,11 +236,17 @@ class SEOHead {
 
         echo "\n<!-- Jio SEO -->\n";
 
-        // Favicon（无站点图标时回退）。
+        // Favicon（无站点图标时回退）；移动端图标走 apple-touch-icon。
         if (function_exists('jio_get_favicon_url')) {
             $favicon = (string) jio_get_favicon_url();
             if ($favicon !== '' && !has_site_icon()) {
                 printf('<link rel="icon" href="%s">' . "\n", esc_url($favicon));
+                if (class_exists('\Jio\Settings\Settings')) {
+                    $mobile_icon = \Jio\Settings\Settings::mobile_icon_url();
+                    if ($mobile_icon !== '') {
+                        printf('<link rel="apple-touch-icon" href="%s">' . "\n", esc_url($mobile_icon));
+                    }
+                }
             }
         }