feat(images): JPEG XL optimizer with webp/jpg fallback and <picture> serving

img_optimize() decodes .jxl with djxl or Imagick, resizes with GD, re-encodes
with cjxl and writes foto.jxl.webp beside it; skips cleanly without a decoder.
Gallery, hero, navbar logos and JSON-LD serve the JXL with the fallback, and
trash/removal move the fallback with its .jxl.

config/functions_site.php

@@ -39,8 +39,7 @@ function site_section_has_content(array $gym, string $sec): bool {
         'equipa'       => (bool) site_team_active($pdo, $gid),
         'horario'      => (bool) site_horario($pdo, '', $gid),
         'mensalidades' => (bool) site_memberships($pdo, $gid),
-        'galeria'      => !empty($gym['gallery_id']) && preg_grep('/\.(jpe?g|png|gif|webp|jxl)$/i',
-                              glob(dirname(__DIR__) . '/uploads/gallery/' . (int) $gym['gallery_id'] . '/*') ?: []),
+        'galeria'      => !empty($gym['gallery_id']) && img_list_dir('uploads/gallery/' . (int) $gym['gallery_id']),
         default        => true,
     };
 }