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/pages/GenetikPage.tsx

@@ -0,0 +1,11 @@
+import { de } from '../strings/de'
+
+export default function GenetikPage() {
+  const t = de.pages.genetik
+  return (
+    <section className="page">
+      <h2>{t.title}</h2>
+      <p className="muted">{t.subtitle}</p>
+    </section>
+  )
+}