Add accessibility toolbar with 6 toggleable options

Adds a floating accessibility toolbar to the template with individually
enable/disable options: text resize, color inversion, high contrast,
highlight links, readable font, and pause animations. Each option has
an admin toggle in the Theme tab and persists visitor preferences in
localStorage. Also fixes count() on null in mod_login override.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/html/mod_login/default.php

@@ -71,7 +71,7 @@ $headerClass = htmlspecialchars($params->get('header_class', ''), ENT_COMPAT, 'U
                 </div>
             </div>
 
-            <?php if (count($twofactormethods) > 1) : ?>
+            <?php if (!empty($twofactormethods) && count($twofactormethods) > 1) : ?>
                 <div class="mod-login__field mb-3">
                     <label for="modlgn-secretkey-<?php echo $module->id; ?>" class="form-label visually-hidden"><?php echo Text::_('JGLOBAL_SECRETKEY'); ?></label>
                     <div class="input-group">