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',
 ];

tests/check-versioned-stage-config.php

@@ -25,6 +25,7 @@ if (!is_array($config)) {
         'bchoice' => ['a' => 1, 'b' => 0, 'default' => 'a'],
         'chb' => ['a' => 1, 'b' => 0, 'default' => 'b'],
         'tp' => ['1' => 0, '2' => 1, 'default' => '1'],
+        'focus' => ['center' => 1, 'face' => 0, 'default' => 'center'],
     ];
 
     if (($config['host'] ?? null) !== $expected['host']) {