feat(seo): sitemap + robots.txt + meta OG/Twitter/canonical + site URL

- Installe @astrojs/sitemap ; integre dans astro.config.mjs avec filtre /api/
- Ajoute site: 'https://trans-former.fr' pour canonical absolues
- BaseLayout : props ogImage + canonical + isArticle + articleDate ; meta description/canonical/robots/OG/Twitter Card complets ; suppression doublons
- manifeste.astro : passe isArticle=true pour Schema.org Article
- public/robots.txt : open index + GPTBot/ClaudeBot/Google-Extended/Applebot-Extended/PerplexityBot explicites

package.json

@@ -16,6 +16,7 @@
   },
   "dependencies": {
     "@astrojs/node": "^10.1.0",
+    "@astrojs/sitemap": "^3.7.2",
     "@astrojs/vue": "^6.0.1",
     "@fontsource-variable/roboto-condensed": "^5.2.8",
     "@tailwindcss/vite": "^4.2.4",