feat(site): root opens the main gym; other gyms as navbar buttons

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -78,7 +78,7 @@ Sistema → Auditoria (`platform.php?tool=audit`).
 Since 2026-09-25, one install serves several physical gyms: the `gyms` table, one URL
 folder per gym (`/3070/`, `/<postal-code>/`), content rows carry `gym_id`, and the
 backoffice has an "a editar" gym switcher (`admin_gym_id()` /
-`admin_gym()` in `config/functions_gyms.php`). `/` and `/home` 301 to the default gym.
+`admin_gym()` in `config/functions_gyms.php`). `/` and `/home` 301 to the default gym (`is_default`); the other published gyms appear as highlighted buttons (`.gym-switch-btn`) at the right end of the public navbar (`site/includes/navbar.php`). `site/gym_chooser.php` is no longer used.
 An unpublished gym 404s publicly but previews for staff.
 
 ## platform-tools