Add cross-domain tracking support (v1.2.0)

Add linker domains setting to Advanced tab for cross-domain measurement.
Outputs gtag('set','linker') before config call so GA4 appends the _gl
parameter to links pointing at configured domains.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/class-att-consent.php

@@ -60,9 +60,10 @@ class ATT_Consent {
 		'cat_marketing_desc'  => 'These cookies are used to deliver advertisements relevant to you.',
 
 		// Advanced.
-		'url_passthrough'    => true,
-		'ads_data_redaction' => true,
-		'wait_for_update'    => 500,
+		'url_passthrough'        => true,
+		'ads_data_redaction'     => true,
+		'wait_for_update'        => 500,
+		'cross_domain_domains'   => '',
 	);
 
 	/**