feat: Add comprehensive SEO implementation

- Add robots.txt with admin/API blocking
- Add dynamic sitemap.xml with static pages and genre pages
- Implement full meta tags (Open Graph, Twitter Cards, Canonical, Alternates)
- Add SEO helper functions for domain detection and URL generation
- Add generateMetadata to all pages (homepage, about, genre, special)
- Support automatic domain detection for hoerdle.de and hördle.de
- Add SEO configuration to lib/config.ts

app/robots.ts

@@ -0,0 +1,20 @@
+import { MetadataRoute } from 'next';
+import { config } from '@/lib/config';
+
+export default function robots(): MetadataRoute.Robots {
+    const baseUrl = process.env.NEXT_PUBLIC_DOMAIN || config.domain;
+    const protocol = process.env.NODE_ENV === 'production' ? 'https' : 'http';
+    const siteUrl = `${protocol}://${baseUrl}`;
+
+    return {
+        rules: [
+            {
+                userAgent: '*',
+                allow: '/',
+                disallow: ['/admin/', '/api/'],
+            },
+        ],
+        sitemap: `${siteUrl}/sitemap.xml`,
+    };
+}
+