feat(branding): re-theme Bootstrap to the brand colour; 3D neon button hover; hide Mensalidades link without plans

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

config/functions_site.php

@@ -36,7 +36,10 @@ function site_navbar(?array $gym = null): array {
     $items = ['Home' => ['#home', null]];
     foreach (['Modalidades' => 'modalidades', 'Equipa' => 'equipa', 'Horário' => 'horario',
               'Mensalidades' => 'mensalidades', 'Contactos' => 'contactos'] as $label => $sec) {
-        if (gym_section_on($gym, $sec)) $items[$label] = ['#' . $sec, $sec];
+        if (!gym_section_on($gym, $sec)) continue;
+        // No active plans → the section isn't rendered, so no link to it either.
+        if ($sec === 'mensalidades' && !empty($GLOBALS['pdo']) && !site_memberships($GLOBALS['pdo'], (int) $gym['id'])) continue;
+        $items[$label] = ['#' . $sec, $sec];
     }
     $nav = [];
     foreach ($items as $label => [$hash]) $nav[$label] = gym_url($gym, $hash);