feat: event reminders + default reminder setting + local notifications (iOS)

Per-event reminders (multiple, local calendars only) in the editor, prefilled
from a new "default reminder" setting that applies to all events otherwise.
CalEvent gains `reminders`; AppSettings/SettingsSync sync default_reminder_minutes
(always group). New NotificationScheduler requests permission and schedules the
soonest ≤60 upcoming reminders via UNUserNotificationCenter, rescheduling on
load/sync/edit and when the default changes (skipped in group overlay).

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

Calendarr iOS/Models/CalendarStore.swift

@@ -264,6 +264,15 @@ class CalendarStore {
             return !hiddenCalendarKeys.contains(key)
                 && !banishedCalendarKeys.contains(key)
         }
+        // Personal events drive local reminder notifications.
+        NotificationScheduler.reschedule(events: allCachedEvents)
+    }
+
+    /// Recompute scheduled reminder notifications from the personal cache
+    /// (skipped while a group overlay is active).
+    func rescheduleNotifications() {
+        guard activeGroup == nil else { return }
+        NotificationScheduler.reschedule(events: allCachedEvents)
     }
 
     /// Optimistically drop a just-deleted event from the cache so it disappears