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/GerbilDetailPage.tsx

@@ -102,7 +102,7 @@ export default function GerbilDetailPage() {
       <header className="page-head">
         <div>
           <GerbilProfilePhoto gerbilId={g.id} />
-          <h2>{g.name}</h2>
+          <h2>{g.name || de.pages.gerbils.nameless}</h2>
           <span className={`badge badge--${g.status.toLowerCase()}`}>{statusLabel(g.status)}</span>
           {g.isResident === false && (
             <span className="badge badge--external" title={de.pages.gerbils.externalTitle}>
@@ -153,7 +153,7 @@ export default function GerbilDetailPage() {
         {g.status === 'Deceased' && (
           <>
             <Row label={t.fields.dateOfDeath} value={formatDate(g.dateOfDeath)} />
-            <Row label={t.fields.causeOfDeath} value={g.causeOfDeath} />
+            <Row label={t.fields.causeOfDeath} value={g.causeOfDeath || null} />
           </>
         )}
         {g.status === 'GivenAway' && (
@@ -190,7 +190,7 @@ export default function GerbilDetailPage() {
           }
         />
         <Row label={t.fields.receiver} value={lookup(contactName, g.receiverContactId)} />
-        <Row label={t.fields.notes} value={g.notes} />
+        <Row label={t.fields.notes} value={g.notes || null} />
       </dl>
 
       <h3>{t.detail.genetics}</h3>