fix: add _APP_DOMAIN_SITES variables.php so the env is initialized

Without this, API calls to Appwrite can result in rule_not_found errors.

app/config/variables.php

@@ -106,6 +106,15 @@ return [
                 'question' => '',
                 'filter' => ''
             ],
+            [
+                'name' => '_APP_DOMAIN_SITES',
+                'description' => 'A domain to use for site preview URLs.',
+                'introduction' => '',
+                'default' => 'sites.localhost',
+                'required' => false,
+                'question' => '',
+                'filter' => ''
+            ],
             [
                 'name' => '_APP_DOMAIN_TARGET',
                 'description' => 'Deprecated since 1.7.0. A DNS A record hostname to serve as a CNAME target for your Appwrite custom domains. You can use the same value as used for the Appwrite \'_APP_DOMAIN\' variable. The default value is \'localhost\'.',