Expand CSS Variables tab to full variable reference and add custom-hero class

- Replace 8-field CSS Variables tab with 21 comprehensive sections covering
  all variables from light.standard.css and dark.standard.css
- New sections: Links, Layout & Spacing, Breakpoints, Bootstrap Semantic
  Palette, Bootstrap State Colors, Alert & List Group Colors, Standard
  Colors/Grays/Opacity, Shadows & Shadow Tokens, Buttons, Cards,
  Component & Plugin Colors, VirtueMart, Gable
- Add custom-hero class to hero.php outer div (always present)
- Both en-GB and en-US language files updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/html/mod_custom/hero.php

@@ -29,7 +29,7 @@ if ($params->get('backgroundimage')) {
 }
 ?>
 
-<div class="mod-custom custom banner-overlay<?php echo $moduleclass ? ' ' . $moduleclass : ''; ?>" id="<?php echo $modId; ?>">
+<div class="mod-custom custom banner-overlay custom-hero<?php echo $moduleclass ? ' ' . $moduleclass : ''; ?>" id="<?php echo $modId; ?>">
     <div class="overlay">
         <?php echo $module->content; ?>
     </div>