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>

src/Settings/AuthReader.php

@@ -20,15 +20,10 @@ trait AuthReader {
         return !self::close_signup();
     }
 
-    /** 主题内置默认图片 URL */
-    private static function default_asset_url(string $filename): string {
-        return get_template_directory_uri() . '/assets/icons/' . $filename;
-    }
-
     /** 获取 Logo URL（留空时使用内置默认） */
     public static function logo_url(): string {
         $url = trim((string) self::get_value('site_logo', ''));
-        return $url !== '' ? $url : self::default_asset_url('default-logo.svg');
+        return $url !== '' ? $url : get_template_directory_uri() . '/assets/img/logo.png';
     }
 
     /** 获取暗色模式 Logo URL（留空时跟随浅色模式） */