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/plugins/kirby-seo/blueprints/fields/meta-group.yml

@@ -0,0 +1,34 @@
+type: group
+fields:
+  _metaHeadline:
+    label: seo.page.meta.headline
+    type: headline
+    numbered: false
+  metaTitle:
+    label: seo.fields.titleOverwrite.label
+    type: seo-writer
+    ai: title
+    placeholder: "{{ page.title }}"
+  metaTemplate:
+    extends: seo/fields/title-template
+    label: seo.fields.metaTitleTemplate.label
+    help: seo.fields.metaTitleTemplate.help
+    width: 2/3
+    placeholder: "{{ page.metadata.metaTemplate }}"
+  useTitleTemplate:
+    label: seo.fields.useTitleTemplate.label
+    type: toggle
+    help: seo.fields.useTitleTemplate.help
+    width: 1/3
+    default: true
+    text:
+      - "{{ t('seo.fields.useTitleTemplate.no') }}"
+      - "{{ t('seo.fields.useTitleTemplate.yes') }}"
+  metaDescription:
+    label: seo.fields.metaDescription.label
+    type: seo-writer
+    ai: description
+    help: seo.fields.metaDescription.help
+    placeholder: "{{ page.metadata.metaDescription }}"
+  _seoLine1:
+    type: line