Add birthday feature (iOS)

- LocalCalendar/CalEvent gain is_birthday + birthday fields; API sends
  rrule/external_uid/birth_year and can create/update birthday calendars
- BirthdaysImporter: mirrors Contacts birthdays into a chosen birthday
  calendar, reconciling by external_uid (leaves manual entries untouched)
- AccountsView: "birthday calendar" toggle + notify-days when creating a
  calendar, and a "Birthdays from Contacts" section (enable + target + sync now)
- FAB long-press menu -> "New birthday" opens a minimal name+date mask
  (BirthdayEditorSheet) targeting birthday calendars, with year-unknown option
- Event bars render display_title (age) and a cake icon via EventLabel
- NSContactsUsageDescription added to both app build configs

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

Calendarr iOS/Views/Calendar/WeekView.swift

@@ -84,7 +84,7 @@ struct WeekView: View {
                 VStack(spacing: 1) {
                     ForEach(dayEvs.prefix(2)) { ev in
                         Button { onEventTap(ev) } label: {
-                            Text(ev.title)
+                            EventLabel(event: ev)
                                 .font(.system(size: 9, weight: .medium))
                                 .foregroundStyle(.white)
                                 .lineLimit(1)