feat: pages contenu + SEO + Umami + sitemap

- Pages /a-propos, /mentions-legales, /dependances, /soutenir (Ko-fi)
- Header/footer enrichis (lien Soutenir, mentions, API publique)
- @astrojs/sitemap + /sitemap-departements.xml.ts (96 dépts)
- robots.txt + og-image.svg + JSON-LD Schema.org
- Meta tags Open Graph + Twitter Card + canonical
- Umami snippet conditionnel (UMAMI_WEBSITE_ID env)
- Slot SENTRY_DSN dans .env.tmpl (GlitchTip — SDK à wire ultérieurement)
- @tailwindcss/typography pour les pages contenu

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tailwind.config.mjs

@@ -1,3 +1,6 @@
+import { createRequire } from 'node:module';
+const require = createRequire(import.meta.url);
+
 /** @type {import('tailwindcss').Config} */
 export default {
   content: ['./src/**/*.{astro,html,js,jsx,md,mdx,svelte,ts,tsx,vue}'],
@@ -24,5 +27,5 @@ export default {
       },
     },
   },
-  plugins: [],
+  plugins: [require('@tailwindcss/typography')],
 };