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/Calendar/CalendarHostView.swift

@@ -535,6 +535,9 @@ struct CalendarHostView: View {
     /// shows up without the user opening the filter sheet.
     private func syncFromServer(force: Bool = false) async {
         await SettingsSync.pull(api: api)
+        // Mirror Contacts birthdays on every server sync (manual, resume,
+        // periodic) — the user expects "sync with server" to include birthdays.
+        if BirthdaysImporter.isEnabled { await BirthdaysImporter.sync(api: api) }
         let changed = await store.reconcileCalendarVisibility(api: api)
         if changed || force { await forceReload() }
     }