- Contacts sync fills only an EXISTING birthday calendar (birthdayCalendar),
never auto-creates it — deleting it no longer resurrects it on sync
- Accounts + menu gains "Geburtstagskalender" (only when none exists) to create
the single birthday calendar explicitly
- the Birthdays-from-Contacts section shows a "create one first" hint when there
is no birthday calendar; enabling sync only requests Contacts access
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>