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/snippets/header.php

@@ -56,8 +56,14 @@
     </button>
 
     <div id="lang-toggle">
-        <button disabled>FR</button>
-        <button><a href="/en">EN</a></button>
+        <?php foreach($kirby->languages() as $language): ?>
+        <a
+            href="<?= $language->url() ?>"
+            <?php e($language->code() === $kirby->language()->code(), 'aria-current="page"') ?>
+        >
+            <?= strtoupper($language->code()) ?>
+        </a>
+        <?php endforeach ?>
     </div>
 
     <button id="menu-toggle">