feat: add change alerts for schedule updates with notifications

ios/Shared/Storage.swift

@@ -35,6 +35,8 @@ struct UserSettings: Codable, Hashable {
     var theme: ThemeMode = .system
     var reminders = false
     var reminderMinutes = 10
+    /** A notification when the background refresh finds changes in the group's document. */
+    var changeAlerts = true
     var recent: [SavedGroup] = []
     var display = Display()
     /** Hidden lessons per group, as Lesson.hideKey values. */
@@ -54,6 +56,7 @@ extension UserSettings {
         theme = try c.decodeIfPresent(ThemeMode.self, forKey: .theme) ?? .system
         reminders = try c.decodeIfPresent(Bool.self, forKey: .reminders) ?? false
         reminderMinutes = try c.decodeIfPresent(Int.self, forKey: .reminderMinutes) ?? 10
+        changeAlerts = try c.decodeIfPresent(Bool.self, forKey: .changeAlerts) ?? true
         recent = try c.decodeIfPresent([SavedGroup].self, forKey: .recent) ?? []
         display = try c.decodeIfPresent(Display.self, forKey: .display) ?? Display()
         hiddenByGroup = try c.decodeIfPresent([String: [String]].self, forKey: .hiddenByGroup) ?? [:]