feat: custom reminder picker, muted-calendar hint, synced default duration

- Reminder editor: presets + custom number+unit (minutes/hours/days/weeks)
- Grey out + footer hint when the selected calendar's reminders are muted;
  reminders are kept, scheduler already skips them
- New synced setting defaultEventDurationMinutes for new events

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

Calendarr iOS/Models/AppSettings.swift

@@ -19,6 +19,7 @@ struct AppSettings: Codable {
     var privateEventVisibility: String = "busy"   // 'hidden' | 'busy'
     var groupVisibleCalendarId: Int? = nil
     var defaultReminderMinutes: Int? = nil         // minutes before start; nil = off
+    var defaultEventDurationMinutes: Int = 60      // applied to a new event's end time
 
     enum CodingKeys: String, CodingKey {
         case defaultView = "default_view"
@@ -39,6 +40,7 @@ struct AppSettings: Codable {
         case privateEventVisibility = "private_event_visibility"
         case groupVisibleCalendarId = "group_visible_calendar_id"
         case defaultReminderMinutes = "default_reminder_minutes"
+        case defaultEventDurationMinutes = "default_event_duration_minutes"
     }
 
     init() {}
@@ -69,6 +71,7 @@ struct AppSettings: Codable {
         privateEventVisibility = try c.decodeIfPresent(String.self, forKey: .privateEventVisibility) ?? d.privateEventVisibility
         groupVisibleCalendarId = try c.decodeIfPresent(Int.self, forKey: .groupVisibleCalendarId)
         defaultReminderMinutes = try c.decodeIfPresent(Int.self, forKey: .defaultReminderMinutes)
+        defaultEventDurationMinutes = try c.decodeIfPresent(Int.self, forKey: .defaultEventDurationMinutes) ?? d.defaultEventDurationMinutes
     }
 }