Fix release notification resource and add multiple advance reminders

mobile/lib/domain/app_settings.dart

@@ -3,23 +3,31 @@ import 'semester.dart';
 enum ColorMode { single, multicolor }
 
 class AppSettings {
-  const AppSettings({
+  AppSettings({
     this.semester,
     this.remindersEnabled = true,
-    this.reminderMinutes = 15,
+    int reminderMinutes = 15,
+    List<int>? reminderOffsets,
     this.colorMode = ColorMode.multicolor,
     this.singleColorArgb = 0xFF348F91,
     this.calendarEnabled = false,
     this.calendarId,
     this.courseHeightScale = defaultCourseHeightScale,
-  });
+  }) : reminderOffsets = List.unmodifiable(
+         (reminderOffsets ?? [reminderMinutes]).toSet().toList()..sort(),
+       );
+  static const allowedReminderMinutes = [5, 10, 15, 30, 60];
   static const defaultCourseHeightScale = 1.25;
   static const minCourseHeightScale = 0.8;
   static const maxCourseHeightScale = 2.0;
   final double courseHeightScale;
   final Semester? semester;
   final bool remindersEnabled, calendarEnabled;
-  final int reminderMinutes, singleColorArgb;
+  final int singleColorArgb;
+  final List<int> reminderOffsets;
+  // Retain the single-value API and JSON field for existing saved data.
+  int get reminderMinutes =>
+      reminderOffsets.isEmpty ? 15 : reminderOffsets.first;
   final ColorMode colorMode;
   final String? calendarId;
   AppSettings copyWith({
@@ -27,6 +35,7 @@ class AppSettings {
     bool clearSemester = false,
     bool? remindersEnabled,
     int? reminderMinutes,
+    List<int>? reminderOffsets,
     ColorMode? colorMode,
     int? singleColorArgb,
     bool? calendarEnabled,
@@ -35,7 +44,9 @@ class AppSettings {
   }) => AppSettings(
     semester: clearSemester ? null : semester ?? this.semester,
     remindersEnabled: remindersEnabled ?? this.remindersEnabled,
-    reminderMinutes: reminderMinutes ?? this.reminderMinutes,
+    reminderOffsets:
+        reminderOffsets ??
+        (reminderMinutes == null ? this.reminderOffsets : [reminderMinutes]),
     colorMode: colorMode ?? this.colorMode,
     singleColorArgb: singleColorArgb ?? this.singleColorArgb,
     calendarEnabled: calendarEnabled ?? this.calendarEnabled,