UI-POLISH-1: namenlose Tiere + Leerwert-Konsistenz

- de.ts: nameless = '(ohne Namen)' (neuer Platzhalter-String)
- GerbilsPage: g.name || t.nameless in Karten-Name + Checkbox-Aria
- GerbilDetailPage: g.name || nameless im h2-Header; g.causeOfDeath/notes via || null (leer-string-sicher wie Herkunft)
- AnimalPicker: g.name || nameless im Picker-Button

e2e: +1 nameless-stub in mock-data; 2 neue Tests (Liste + Detail-Header)

Gate: vitest 82/82, e2e 130/130, tsc clean

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

@@ -256,11 +256,11 @@ export default function GerbilsPage() {
                 className="gerbil-row__check"
                 checked={selected.has(g.id)}
                 onChange={() => toggleSelect(g.id)}
-                aria-label={g.name}
+                aria-label={g.name || t.nameless}
               />
               <Link to={`/rennmaeuse/${g.id}`} className="gerbil-card">
                 <span className="gerbil-card__name">
-                  {g.name}
+                  {g.name || t.nameless}
                   {g.isResident === false && (
                     <span className="badge badge--external" title={t.externalTitle}>
                       {t.externalBadge}