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>

assets/admin/src/schema/sections/global/logo.js

@@ -1,3 +1,8 @@
+const themeRoot = typeof window !== 'undefined' && window.jioAdminData?.themeUri
+  ? String(window.jioAdminData.themeUri).replace(/\/$/, '')
+  : ''
+const defaultFavicon = themeRoot ? `${themeRoot}/assets/img/favicon.png` : ''
+
 export const logoSchema = {
   key: 'global.logo',
   title: 'LOGO & 品牌',
@@ -6,8 +11,8 @@ export const logoSchema = {
   defaults: {
     site_logo: '',
     site_logo_dark: '',
-    site_favicon: '',
-    site_icon: '',
+    site_favicon: defaultFavicon,
+    site_icon: defaultFavicon,
     logo_width: 150,
     logo_text: '',
   },
@@ -28,8 +33,8 @@ export const logoSchema = {
       title: '站点图标',
       icon: 'fa fas fa-id-badge',
       fields: [
-        { key: 'site_favicon', label: '浏览器图标 (Favicon)', type: 'media', help: '建议 32x32 或 16x16 的 ICO/PNG 文件' },
-        { key: 'site_icon', label: '移动端图标', type: 'media', help: '建议 192x192 的 PNG，用于移动端添加到桌面' },
+        { key: 'site_favicon', label: '浏览器图标 (Favicon)', type: 'media', default: defaultFavicon, help: '建议 32x32 或 16x16 的 ICO/PNG 文件' },
+        { key: 'site_icon', label: '移动端图标', type: 'media', default: defaultFavicon, help: '建议 192x192 的 PNG，用于移动端添加到桌面' },
       ],
     },
   ],