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/docs/1_features/09_utm-share.md

@@ -0,0 +1,42 @@
+---
+title: UTM Share
+intro: Share links with tracking parameters for your marketing campaigns
+---
+
+When you share a link to your site in a newsletter, a social media post, or an ad, you want to know which links actually bring in traffic. UTM parameters are tags you add to a URL so analytics tools like Google Analytics can tell you exactly where a visitor came from.
+
+A URL with UTM parameters looks like this:
+
+```
+https://example.com/blog/my-post?utm_source=newsletter&utm_medium=email&utm_campaign=spring-sale
+```
+
+Kirby SEO adds a **UTM Share** button to your page views. Click it to open a dialog where you can fill in the parameters and copy the resulting URL.
+
+![UTM Share dialog in the Panel](utm-share.png)
+
+The dialog has five standard UTM parameters:
+
+- `utm_source`: where the traffic comes from (e.g. `google`, `newsletter`)
+- `utm_medium`: the type of channel (e.g. `cpc`, `email`, `social`)
+- `utm_campaign`: the name of the campaign (e.g. `spring_sale`)
+- `utm_content`: to tell apart different links in the same campaign (e.g. `logo_link`)
+- `utm_term`: the keyword, for paid search ads (e.g. `running shoes`)
+
+You don't need all five. Most of the time, `utm_source`, `utm_medium`, and `utm_campaign` are enough.
+
+There's also a `ref` field. This is not part of the UTM standard, but many analytics tools (like Plausible and Pirsch) use it as a lightweight way to track the referring site.
+
+To add the button to your page blueprints:
+
+```yaml
+# site/blueprints/pages/default.yml
+buttons:
+  - open
+  - preview
+  - "-"
+  - settings
+  - languages
+  - status
+  - utm-share
+```