feat(i18n): configure French as default language without URL prefix

- Set French as default language with URL '/' (no prefix)
- Set English language with URL '/en'
- Replace hardcoded language buttons with dynamic links
- Fix invalid HTML (remove <a> inside <button>)
- Add aria-current attribute for accessibility

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

site/languages/fr.php

@@ -6,4 +6,5 @@ return [
   'direction' => 'ltr',
   'locale'    => 'fr_FR.UTF-8',
   'name'      => 'Français',
+  'url'       => '/',
 ];