Birthdays: explicit calendar creation, sync never auto-creates (iOS)

- 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>

Calendarr iOS/Services/BirthdaysImporter.swift

@@ -141,7 +141,10 @@ enum BirthdaysImporter {
         guard isEnabled else { return }
         guard await requestAccess() else { return }
         guard let contacts = try? readContactBirthdays() else { return }
-        guard let cal = await ensureBirthdayCalendar(api: api) else { return }
+        // Only fill an EXISTING birthday calendar — never auto-create it. The
+        // calendar is created explicitly; deleting it means sync has nowhere to
+        // go (and must not resurrect it).
+        guard let cal = await birthdayCalendar(api: api) else { return }
         guard let existing = try? await api.getBirthdayEntries(calendarId: cal.id) else { return }
 
         // Only reconcile THIS device's contact rows; leave other devices'