chore(assets): remove dead icon assets and consolidate icons/

Delete zero-referenced leftovers (31 img SVGs, duplicate top-level
vip badges, favicon/icon/person_background pngs, assets/fonts/,
tmp-debug/, gutenberg-extend.js and the FA eot/ttf/svg font formats)
and give icon-purpose SVGs one home in assets/icons/ (upload-add,
default favicon/logo, playing, user-certification) with every reference
updated. Icon SVG files were audited by name across PHP/CSS/JS before
deletion.

src/Settings/AuthReader.php

@@ -22,7 +22,7 @@ trait AuthReader {
 
     /** 主题内置默认图片 URL */
     private static function default_asset_url(string $filename): string {
-        return get_template_directory_uri() . '/assets/img/' . $filename;
+        return get_template_directory_uri() . '/assets/icons/' . $filename;
     }
 
     /** 获取 Logo URL（留空时使用内置默认） */