feat: SEO enhancements — server metadata, sitemap, robots, dynamic base URL

- Split all page.tsx files into server wrapper (metadata export) + client.tsx (Apollo/interactive)
- Add robots.ts and sitemap.ts (tournaments, teams, players)
- Add metadataBase, OpenGraph and Twitter card metadata to root layout
- Replace hardcoded worldcup.pivoine.art with NEXT_PUBLIC_SITE_URL env var (sitemap/robots) and relative paths (page metadata, resolved by Next.js against metadataBase)

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

app/robots.ts

@@ -0,0 +1,8 @@
+import type { MetadataRoute } from 'next'
+
+export default function robots(): MetadataRoute.Robots {
+  return {
+    rules: { userAgent: '*', allow: '/' },
+    sitemap: `${(process.env.NEXT_PUBLIC_SITE_URL ?? 'http://localhost:3000').replace(/\/$/, '')}/sitemap.xml`,
+  }
+}