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/en.php

@@ -5,4 +5,5 @@ return [
   'direction' => 'ltr',
   'locale'    => 'en_US.UTF-8',
   'name'      => 'English',
+  'url'       => '/en',
 ];

site/languages/fr.php

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

site/snippets/header.php

@@ -56,8 +56,14 @@
     </button>
 
     <div id="lang-toggle">
-        <button disabled>FR</button>
+        <?php foreach($kirby->languages() as $language): ?>
-        <button><a href="/en">EN</a></button>
+        <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">