Birthdays: single calendar model + sync on server-sync (iOS)

- one birthday calendar per user; BirthdaysImporter auto-creates/targets it
  (no picker) and reconciles per-device (external_uid contact:<deviceId>:<id>)
  so multiple devices don't clash
- Contacts birthday sync now runs on every "Sync with server" (syncFromServer),
  not just app launch and the manual button
- report the device after each sync (POST /api/birthdays/sync-report) for the
  web device list
- Accounts: Birthdays-from-Contacts section drops the target picker, keeps the
  reminder picker; removed the birthday toggle from the generic new-calendar sheet
- New-birthday sheet targets the single calendar, offers activation if none
- exclude the birthday calendar from the group-visible ("shared calendar") picker

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

Calendarr iOS/Views/SettingsView.swift

@@ -205,7 +205,9 @@ struct SettingsView: View {
             groupVisibleId = s.groupVisibleCalendarId ?? 0
         }
         if let cals = try? await api.getLocalCalendars() {
-            ownLocalCals = cals.filter { $0.owned && !$0.group }
+            // A birthday calendar may be shared directly, but never stand in as
+            // the group-visible personal calendar.
+            ownLocalCals = cals.filter { $0.owned && !$0.group && !$0.isBirthday }
         }
     }