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>

includes/auth_header.php

@@ -29,6 +29,7 @@ $app_name   = defined('APP_NAME') ? APP_NAME : 'FitMe 3070';
     })();
     </script>
     <link href="<?= base_url('assets/vendor/bootstrap5/css/bootstrap.min.css') ?>" rel="stylesheet">
+    <link href="<?= htmlspecialchars(_asset_versioned('assets/css/bs-brand.css'), ENT_QUOTES) ?>" rel="stylesheet">
     <link href="<?= base_url('assets/vendor/bootstrap-icons/bootstrap-icons.css') ?>" rel="stylesheet">
     <style>
         :root {