FORM-FIELDS-1: originBreeder + isDeaf in GerbilFormPage

- types.ts: isDeaf?: boolean|null in Gerbil + CreateGerbil (UpdateGerbil erbt via Partial)
- de.ts: fields.originBreeder, fields.isDeaf; form.isDeafUnknown/Yes/No
- GerbilFormPage: originBreeder Freitext-Input (neben originContactId-Dropdown); isDeaf Tristate-Select (Unbekannt/Ja/Nein); round-trip load→display→save

e2e: 2 neue Tests (originBreeder Freitext + isDeaf Tristate round-trip)

Gate: vitest 82/82, e2e 146/146, tsc clean
Rebase auf Pams CR-2 folgt nach deren Merge.

gerbil-manager-web/src/api/types.ts

@@ -54,6 +54,8 @@ export interface Gerbil {
    * as resident (backend default true).
    */
   isResident?: boolean
+  /** FORM-FIELDS-1: true=gehörlos, false=hörend, null=unbekannt. */
+  isDeaf?: boolean | null
 }
 
 /** Payload for POST /gerbils. */
@@ -76,6 +78,7 @@ export interface CreateGerbil {
   characterTraits?: string[] | null
   characterNote?: string | null
   isResident?: boolean
+  isDeaf?: boolean | null
 }
 
 /** Payload for PUT /gerbils/{id} (all optional / partial update). */