feat: intégration plugin Kirby SEO

- Ajout de tobimori/kirby-seo via Composer
- snippet('seo/head') dans header.php (remplace les meta manuels)
- snippet('seo/schemas') dans footer.php pour JSON-LD
- Onglet SEO ajouté dans site.yml et tous les blueprints de pages
- Configuration SEO dans config.php (sitemap, robots, canonicalBase TODO)

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

site/snippets/footer.php

@@ -1,2 +1,3 @@
+<?php snippet('seo/schemas') ?>
 </body>
 </html>

site/snippets/header.php

@@ -4,21 +4,11 @@
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
 
-  <title><?= $site->title() ?> - <?= $page->title() ?></title>
-  <meta name="description" content="<?= $page->meta_description()->or($site->site_description()) ?>">
+  <?php snippet('seo/head') ?>
 
   <!-- Favicon -->
   <link rel="icon" type="image/png" href="<?= url('assets/favicon.png') ?>">
 
-  <!-- Open Graph -->
-  <meta property="og:title" content="<?= $page->meta_title()->or($page->title()) ?>">
-  <meta property="og:description" content="<?= $page->meta_description()->or($site->site_description()) ?>">
-  <meta property="og:type" content="website">
-  <meta property="og:url" content="<?= $page->url() ?>">
-  <?php if ($cover = $page->cover()->toFile()): ?>
-    <meta property="og:image" content="<?= $cover->url() ?>">
-  <?php endif ?>
-
   <?php if (file_exists('assets/dist')): ?>
     <!-- Production: Load compiled assets -->
     <script type="module" src="<?= url('assets/dist/index.js') ?>" defer></script>