BESTAND-FILTER follow-up: residency toggle on the Gerbil create/edit form

Completes residency end-to-end: a 'Gehört zum eigenen Bestand' checkbox on the
Rennmaus-Formular (default on for new animals) lets the user mark a tier as an
external ancestor by hand — previously isResident was only set at import. Wired
through FormState + formFromGerbil (defaults true) + the CreateGerbil payload.

German via de.ts (LF). e2e: edit Krümel, untick Bestand, save -> detail shows
the 'Extern' badge (proves form field -> payload -> persistence). Gate green:
build, eslint, vitest 78, e2e 86.

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

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

@@ -24,6 +24,7 @@ interface FormState {
   receiverContactId: string
   genotype: string
   notes: string
+  isResident: boolean
 }
 
 const EMPTY: FormState = {
@@ -41,6 +42,7 @@ const EMPTY: FormState = {
   receiverContactId: '',
   genotype: '',
   notes: '',
+  isResident: true,
 }
 
 function formFromGerbil(g: {
@@ -58,6 +60,7 @@ function formFromGerbil(g: {
   receiverContactId: string | null
   genotype: string | null
   notes: string | null
+  isResident?: boolean | null
 }): FormState {
   return {
     name: g.name,
@@ -74,6 +77,7 @@ function formFromGerbil(g: {
     receiverContactId: g.receiverContactId ?? '',
     genotype: g.genotype ?? '',
     notes: g.notes ?? '',
+    isResident: g.isResident ?? true,
   }
 }
 
@@ -166,6 +170,7 @@ export default function GerbilFormPage() {
       receiverContactId: nn(form.receiverContactId),
       genotype: nn(form.genotype),
       notes: nn(form.notes),
+      isResident: form.isResident,
     }
     const result = await mutation.run(body)
     if (result.ok) navigate(`/rennmaeuse/${result.value.id}`)
@@ -355,6 +360,15 @@ export default function GerbilFormPage() {
           <textarea value={form.notes} onChange={(e) => set('notes', e.target.value)} />
         </label>
 
+        <label className="field field--check" title={t.form.isResidentHint}>
+          <span>{t.form.isResidentLabel}</span>
+          <input
+            type="checkbox"
+            checked={form.isResident}
+            onChange={(e) => set('isResident', e.target.checked)}
+          />
+        </label>
+
         {mutation.error && <div className="alert alert--error">{mutation.error}</div>}
 
         <div className="form-actions">