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/Models/Localization.swift

@@ -347,6 +347,9 @@ private let strings: [String: [String: String]] = [
 
         // Birthdays
         "birthday.new": "Neuer Geburtstag",
+        "birthday.calendar_name": "Geburtstage",
+        "birthday.activate": "Geburtstagskalender aktivieren",
+        "birthday.activate_hint": "Aktiviere den Geburtstagskalender, um Geburtstage anzulegen. Er erscheint als eigener Kalender in der Seitenleiste.",
         "birthday.person": "Name",
         "birthday.person_placeholder": "Name der Person",
         "birthday.date": "Geburtstag",
@@ -702,6 +705,9 @@ private let strings: [String: [String: String]] = [
 
         // Birthdays
         "birthday.new": "New birthday",
+        "birthday.calendar_name": "Birthdays",
+        "birthday.activate": "Enable birthday calendar",
+        "birthday.activate_hint": "Enable the birthday calendar to add birthdays. It appears as its own calendar in the sidebar.",
         "birthday.person": "Name",
         "birthday.person_placeholder": "Person's name",
         "birthday.date": "Birthday",