修正 fa fab 的字体优先级，微信、QQ、微博等图标现在使用 Brands 字体。

tests/widget-system/page-display-save-harness.php

@@ -167,6 +167,8 @@ namespace {
 	$header_clean = $provider->sanitize_payload('page_display', $header_payload, 'header-components');
 
 	$footer_payload = [
+		// Empty means no preset background; it must remain a valid optional palette value.
+		'footer_theme_bg' => '',
 		'fcode_t1_code' => '<strong>Allowed</strong><script>bad()</script>',
 		'footer_contact_weibo' => 'javascript:alert(1)',
 		'footer_mini_img' => [[