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/components/AnimalPicker.tsx

@@ -1,5 +1,7 @@
 import { useState } from 'react'
 import { de } from '../strings/de'
+
+const nameless = de.pages.gerbils.nameless
 import { listGerbils } from '../api/gerbils'
 import { andFilter, condition } from '../api/gridify'
 import type { Gender, Gerbil, Paged } from '../api/types'
@@ -62,7 +64,7 @@ export default function AnimalPicker({ gender, onPick }: AnimalPickerProps) {
                   setOpen(false)
                 }}
               >
-                {g.name}
+                {g.name || nameless}
               </button>
             </li>
           ))}