Fix release notification resource and add multiple advance reminders

mobile/lib/domain/app_snapshot.dart

@@ -17,14 +17,15 @@ class ImportMetadata {
 
 class AppSnapshot {
   AppSnapshot({
-    this.settings = const AppSettings(),
+    AppSettings? settings,
     List<Course> courses = const [],
     this.importMetadata,
     this.examSchedule,
     SchoolTerm? schoolTerm,
     this.calendarCleanupPending = false,
     Map<String, String> calendarEventIds = const {},
-  }) : schoolTerm = schoolTerm ?? _legacyTerm(courses, examSchedule),
+  }) : settings = settings ?? AppSettings(),
+       schoolTerm = schoolTerm ?? _legacyTerm(courses, examSchedule),
        courses = List.unmodifiable(courses),
        calendarEventIds = Map.unmodifiable(calendarEventIds);
   final AppSettings settings;
@@ -85,6 +86,7 @@ class AppSnapshot {
             },
       'remindersEnabled': settings.remindersEnabled,
       'reminderMinutes': settings.reminderMinutes,
+      'reminderOffsets': settings.reminderOffsets,
       'colorMode': settings.colorMode.name,
       'singleColorArgb': settings.singleColorArgb,
       'calendarEnabled': settings.calendarEnabled,
@@ -112,10 +114,17 @@ class AppSnapshot {
         throw const FormatException('保存课表无效');
       }
       final s = Map<String, dynamic>.from(json['settings'] as Map);
-      final minutes = s['reminderMinutes'] as int;
-      if (![5, 10, 15, 30, 60].contains(minutes)) {
+      final rawOffsets = s.containsKey('reminderOffsets')
+          ? s['reminderOffsets']
+          : [s['reminderMinutes']];
+      if (rawOffsets is! List ||
+          rawOffsets.isEmpty ||
+          rawOffsets.any(
+            (v) => v is! int || !AppSettings.allowedReminderMinutes.contains(v),
+          )) {
         throw const FormatException('提醒时间无效');
       }
+      final offsets = rawOffsets.cast<int>();
       final color = s['singleColorArgb'] as int;
       if (color < 0 || color > 0xFFFFFFFF) throw const FormatException('颜色无效');
       final rawHeight = s.containsKey('courseHeightScale')
@@ -152,7 +161,7 @@ class AppSnapshot {
         settings: AppSettings(
           semester: term,
           remindersEnabled: s['remindersEnabled'] as bool,
-          reminderMinutes: minutes,
+          reminderOffsets: offsets,
           colorMode: ColorMode.values.byName(s['colorMode'] as String),
           singleColorArgb: color,
           calendarEnabled: s['calendarEnabled'] as bool,