FEAT-5: add /genetik route + nav entry + German strings (page stub)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gerbil-manager-web/src/components/AppShell.tsx

@@ -6,6 +6,7 @@ const navItems = [
   { to: '/rennmaeuse', label: de.nav.gerbils, icon: '🐭', end: false },
   { to: '/wuerfe', label: de.nav.litters, icon: '🍼', end: false },
   { to: '/zuechter', label: de.nav.breeders, icon: '🧑‍🌾', end: false },
+  { to: '/genetik', label: de.nav.genetics, icon: '🧬', end: false },
 ]
 
 /**