FEAT-4: install react-d3-tree + /rennmaeuse/:id/stammbaum route (page scaffold)

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

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

@@ -0,0 +1,44 @@
+/**
+ * FEAT-4: Stammbaum (Ahnentafel) — interaktiver Stammbaum-Viewer.
+ *
+ * Erster Ausbauschritt: Route + Grundgerüst (Tier laden, Titel, Rücksprung).
+ * Der eigentliche Viewer (react-d3-tree, Druckansicht, Inzuchtkoeffizient)
+ * folgt in den nächsten FEAT-4-Commits.
+ */
+import { Link, useParams } from 'react-router-dom'
+import { de } from '../strings/de'
+import { getGerbil } from '../api/gerbils'
+import { useApi } from '../hooks/useApi'
+
+export default function StammbaumPage() {
+  const t = de.pages.stammbaum
+  const { id = '' } = useParams()
+
+  const gerbil = useApi(() => getGerbil(id), [id])
+
+  if (gerbil.loading) return <p className="muted">{de.common.loading}</p>
+  if (gerbil.error || !gerbil.data) {
+    return (
+      <section className="page">
+        <p className="muted">{gerbil.error ?? t.notFound}</p>
+        <Link to="/rennmaeuse" className="btn">
+          {de.pages.gerbils.detail.back}
+        </Link>
+      </section>
+    )
+  }
+
+  const g = gerbil.data
+  return (
+    <section className="page">
+      <header className="page-head">
+        <h2>{t.titleFor(g.name)}</h2>
+        <div className="head-actions">
+          <Link to={`/rennmaeuse/${g.id}`} className="btn">
+            {t.backToAnimal}
+          </Link>
+        </div>
+      </header>
+    </section>
+  )
+}