FEAT-4: add Stammbaum button to animal detail head-actions

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

gerbil-manager-web/src/pages/GerbilDetailPage.tsx

@@ -89,6 +89,9 @@ export default function GerbilDetailPage() {
           <Link to={`/genetik?parent=${g.id}`} className="btn">
             {t.detail.testMating}
           </Link>
+          <Link to={`/rennmaeuse/${g.id}/stammbaum`} className="btn">
+            {de.pages.stammbaum.openButton}
+          </Link>
           <Link to="/rennmaeuse" className="btn">
             {t.detail.back}
           </Link>