feat(utm): propagate join focus mode

tests/check-utm-redirect-contract.php

@@ -38,6 +38,7 @@ $expected = [
     'bchoice' => 'a',
     'chb' => 'b',
     'tp' => '1',
+    'focus' => 'center',
     'utm_source' => 'stage-ad',
     'subid' => 'click-42',
 ];