feat: iOS Datenebene + Ersteller-Anzeige + Privat-Flag

- Modelle: CalEvent (creator, isPrivate, owner, isGroupEvent, displayColor),
  LocalCalendar (owned/sharedBy/permission/group), AppSettings
  (privateEventVisibility, groupVisibleCalendarId), UserProfile (displayName);
  neue Modelle CalGroup/GroupMember/DirectoryUser/CalendarShare.
- API: Profil-Update (Name/Login), Sharing-CRUD, Gruppen-CRUD + combined,
  Mitglieder-Farbe, iCal Import (multipart) & Export, private-Flag bei Events.
- Event-Detail zeigt "Erstellt von" (wenn != ich) + Privat-Hinweis;
  Editor hat Privat-Toggle (nur lokale Kalender). Login speichert userId.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Calendarr iOS/Models/Localization.swift

@@ -201,6 +201,7 @@ private let strings: [String: [String: String]] = [
         // Event editor
         "event.title_placeholder": "Titel",
         "event.allday": "Ganztägig",
+        "event.private": "Privat",
         "event.start": "Start",
         "event.end": "Ende",
         "event.location": "Ort",
@@ -462,6 +463,7 @@ private let strings: [String: [String: String]] = [
         // Event editor
         "event.title_placeholder": "Title",
         "event.allday": "All-day",
+        "event.private": "Private",
         "event.start": "Start",
         "event.end": "End",
         "event.location": "Location",