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/Models/Localization.swift

@@ -292,6 +292,7 @@ private let strings: [String: [String: String]] = [
         "accounts.loading": "Lade Konten…",
         "accounts.add.caldav": "CalDAV-Konto",
         "accounts.add.local": "Lokaler Kalender",
+        "accounts.add.birthday": "Geburtstagskalender",
         "accounts.add.ical": "iCal-URL abonnieren",
         "accounts.add.ha": "Home Assistant",
         "accounts.caldav.header": "CalDAV-Konten",
@@ -650,6 +651,7 @@ private let strings: [String: [String: String]] = [
         "accounts.loading": "Loading accounts…",
         "accounts.add.caldav": "CalDAV account",
         "accounts.add.local": "Local calendar",
+        "accounts.add.birthday": "Birthday calendar",
         "accounts.add.ical": "Subscribe to iCal URL",
         "accounts.add.ha": "Home Assistant",
         "accounts.caldav.header": "CalDAV accounts",