extend admin auth cookie lifetime from session-only to 1 week (604800s)

app/src/AdminAuth.php

@@ -30,7 +30,7 @@ class AdminAuth
         }
         // Harden session cookie (item #8)
         session_set_cookie_params([
-            'lifetime' => 0,
+            'lifetime' => 604800,
             'path'     => '/admin',
             'secure'   => (php_sapi_name() !== 'cli-server'),
             'httponly' => true,