feat(branding): default both site logos to assets/img/logo.png

- Point 站点 Logo / 暗色模式 Logo schema defaults (PHP + VUE) and the
  Settings::logo_url() fallback at assets/img/logo.png.
- Point the footer night image default at the same file.
- Remove logo_dark.png and the now-unreferenced assets/icons/default-logo.svg
  along with the private default_asset_url() helper that was its only reader.

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

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

@@ -1,6 +1,7 @@
 const themeRoot = typeof window !== 'undefined' && window.jioAdminData?.themeUri
   ? String(window.jioAdminData.themeUri).replace(/\/$/, '')
   : ''
+const defaultLogo = themeRoot ? `${themeRoot}/assets/img/logo.png` : ''
 const defaultFavicon = themeRoot ? `${themeRoot}/assets/img/favicon.png` : ''
 
 export const logoSchema = {
@@ -9,8 +10,8 @@ export const logoSchema = {
   icon: 'fa fas fa-image',
   description: '管理站点 Logo、站点图标和品牌标识。',
   defaults: {
-    site_logo: '',
-    site_logo_dark: '',
+    site_logo: defaultLogo,
+    site_logo_dark: defaultLogo,
     site_favicon: defaultFavicon,
     site_icon: defaultFavicon,
     logo_text: '',
@@ -21,8 +22,8 @@ export const logoSchema = {
       title: 'Logo 设置',
       icon: 'fa fas fa-image',
       fields: [
-        { key: 'site_logo', label: '站点 Logo', type: 'media', help: '建议尺寸 300x80，支持 PNG/SVG 透明背景' },
-        { key: 'site_logo_dark', label: '暗色模式 Logo', type: 'media', help: '暗色模式下显示的 Logo，留空则使用默认 Logo' },
+        { key: 'site_logo', label: '站点 Logo', type: 'media', default: defaultLogo, help: '建议尺寸 300x80，支持 PNG/SVG 透明背景' },
+        { key: 'site_logo_dark', label: '暗色模式 Logo', type: 'media', default: defaultLogo, help: '暗色模式下显示的 Logo，留空则使用默认 Logo' },
         { key: 'logo_text', label: 'Logo 文字', type: 'input', placeholder: '当无 Logo 图片时显示的文字', help: '同时作为首页标题和 alt 文本使用' },
       ],
     },