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/CalEvent.swift

@@ -41,6 +41,8 @@ struct CalEvent: Identifiable, Hashable {
     // Server-decorated title for the group combined view (group icon / owner
     // prefix); rendered in group mode while `title` stays raw for editing.
     var displayTitle: String? = nil
+    // Reminder offsets in minutes-before-start (0 = at start). Local events only.
+    var reminders: [Int] = []
 
     // Group view supplies a server-resolved colour; otherwise per-event then calendar colour.
     var effectiveColor: String { displayColor ?? color ?? calendarColor }
@@ -82,7 +84,8 @@ struct CalEvent: Identifiable, Hashable {
             owner: EventPerson.from(json["owner"]),
             isGroupEvent: json["is_group_event"] as? Bool ?? false,
             displayColor: (json["display_color"] as? String).flatMap { $0.isEmpty ? nil : $0 },
-            displayTitle: (json["display_title"] as? String).flatMap { $0.isEmpty ? nil : $0 }
+            displayTitle: (json["display_title"] as? String).flatMap { $0.isEmpty ? nil : $0 },
+            reminders: (json["reminders"] as? [Int]) ?? (json["reminders"] as? [Any])?.compactMap { ($0 as? Int) ?? Int("\($0)") } ?? []
         )
     }
 }