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/Settings/FacadeBulk.php

@@ -222,7 +222,15 @@ trait FacadeBulk {
         }
 
         $site_icon = get_site_icon_url(512);
-        return $site_icon ? (string) $site_icon : self::default_asset_url('default-favicon.svg');
+        return $site_icon ? (string) $site_icon : get_template_directory_uri() . '/assets/img/favicon.png';
+    }
+
+
+
+    /** 移动端添加到桌面的图标 URL（留空时跟随浏览器图标） */
+    public static function mobile_icon_url(): string {
+        $custom = trim((string) self::get_value('site_icon', ''));
+        return $custom !== '' ? $custom : self::favicon_url();
     }