Give the leave-wish test fixture its new field

A local literal that predates the typed factories still had to be
updated by hand when wants_dubbeldag reached the frontend.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ei7gCZzdd751uZtPzb1Pep

frontend/src/hooks/useLeaveWishes.test.ts

@@ -24,6 +24,7 @@ const mockWish = {
   parentMode: 'either' as const,
   parentPersonalDataUuid: null,
   windowMode: 'fixed' as const,
+  wantsDubbeldag: false,
   createdAt: '2026-01-01',
   updatedAt: '2026-01-01',
 }