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

lib/seo.ts

@@ -0,0 +1,43 @@
+import { headers } from 'next/headers';
+import { config } from './config';
+
+/**
+ * Get the current base URL from request headers
+ * Automatically detects hoerdle.de or hördle.de (xn--hrdle-jua.de)
+ */
+export async function getBaseUrl(): Promise<string> {
+    const headersList = await headers();
+    const host = headersList.get('host') || headersList.get('x-forwarded-host') || '';
+    
+    let domain = config.domain; // Default fallback
+    
+    if (host) {
+        // Extract domain from host (remove port if present)
+        const detectedDomain = host.split(':')[0].toLowerCase();
+        
+        // Map domains
+        if (detectedDomain === 'hoerdle.de') {
+            domain = 'hoerdle.de';
+        } else if (detectedDomain === 'hördle.de' || detectedDomain === 'xn--hrdle-jua.de') {
+            domain = 'hördle.de';
+        } else {
+            // Use detected domain if it's different from default
+            domain = detectedDomain;
+        }
+    }
+    
+    // Always use HTTPS in production
+    const protocol = process.env.NODE_ENV === 'production' ? 'https' : 'http';
+    return `${protocol}://${domain}`;
+}
+
+/**
+ * Get base URL synchronously (for use in non-async contexts)
+ * Uses environment variable or config as fallback
+ */
+export function getBaseUrlSync(): string {
+    const domain = process.env.NEXT_PUBLIC_DOMAIN || config.domain;
+    const protocol = process.env.NODE_ENV === 'production' ? 'https' : 'http';
+    return `${protocol}://${domain}`;
+}
+