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/AgendaView.swift

@@ -79,7 +79,7 @@ private struct AgendaEventRow: View {
                 .frame(width: 4, height: 40)
 
             VStack(alignment: .leading, spacing: 3) {
-                Text(event.title)
+                EventLabel(event: event)
                     .font(.body.weight(.medium))
                     .foregroundStyle(.primary)
                 HStack(spacing: 6) {