fix: initialize reminder channel and clarify notification permission diagnostics

mobile/lib/services/local_notification_service.dart

@@ -8,6 +8,7 @@ import 'package:flutter/services.dart';
 import '../domain/course.dart';
 import '../domain/app_settings.dart';
 import '../domain/course_occurrence.dart';
+import 'diagnostic_log.dart';
 import 'notification_service.dart';
 
 class ReminderPermissions {
@@ -72,7 +73,8 @@ class LocalNotificationService extends NotificationService {
       await cancelOwned();
       return const ReminderResult(
         ReminderStatus.permissionDenied,
-        message: '请允许通知，并检查系统中的“课前提醒”通知类别。',
+        message:
+            '通知未获允许，当前没有安排课前提醒。请在“通知设置”开启TPSchedule的通知总开关和“课前提醒”类别，再点“检查提醒”。',
       );
     }
     final upcoming =
@@ -144,7 +146,7 @@ class LocalNotificationService extends NotificationService {
     if (!saved.containsAll(desired.keys)) {
       throw const ReminderSetupException(
         'schedule_not_saved',
-        '系统未保存全部提醒，请点击“检查提醒”后重试。',
+        '部分提醒尚未完成登记，请点击“检查提醒”后重试。',
       );
     }
     final message = [
@@ -191,6 +193,12 @@ class PluginReminderGateway extends ReminderGateway {
   final void Function(String key)? onTap;
   final _plugin = FlutterLocalNotificationsPlugin();
   static const _system = MethodChannel('com.tpschedule.app/reminders');
+  static const _courseChannel = AndroidNotificationChannel(
+    'hepan_lessons',
+    '课前提醒',
+    description: '课程开始前的提醒',
+    importance: Importance.high,
+  );
   String? _activeQueryWarning;
   Future<void>? _initializing;
   Future<void> initialize() => _initializing ??= Future<void>(() async {
@@ -205,6 +213,9 @@ class PluginReminderGateway extends ReminderGateway {
       if (ready != true) {
         throw StateError('通知服务初始化失败');
       }
+      // Make the category visible before checking access or scheduling.
+      // Reusing its ID preserves the user's existing Android channel choices.
+      await _android?.createNotificationChannel(_courseChannel);
       final launch = await _plugin.getNotificationAppLaunchDetails();
       if (launch?.didNotificationLaunchApp == true) {
         _select(launch?.notificationResponse?.payload);
@@ -239,9 +250,20 @@ class PluginReminderGateway extends ReminderGateway {
               channel.id == 'hepan_lessons' &&
               channel.importance == Importance.none,
         );
+    final appEnabled = await _android?.areNotificationsEnabled() ?? false;
+    DiagnosticLog.instance.add(
+      'reminder_access_checked',
+      fields: {
+        'result': appEnabled && courseChannelEnabled,
+        'code': !appEnabled
+            ? 'app_notifications_disabled'
+            : !courseChannelEnabled
+            ? 'course_channel_disabled'
+            : 'allowed',
+      },
+    );
     return ReminderPermissions(
-      (await _android?.areNotificationsEnabled() ?? false) &&
-          courseChannelEnabled,
+      appEnabled && courseChannelEnabled,
       await _android?.canScheduleExactNotifications() ?? false,
     );
   }
@@ -305,7 +327,7 @@ class PluginReminderGateway extends ReminderGateway {
       );
       final saved = await _plugin.pendingNotificationRequests();
       if (!saved.any((item) => item.id == -1002)) {
-        throw const ReminderSetupException('test_not_saved', '系统未保存测试提醒。');
+        throw const ReminderSetupException('test_not_saved', '测试提醒登记未完成。');
       }
       return '已安排测试，请按手机 Home 键回到桌面，10 秒后查看通知栏。';
     }
@@ -327,6 +349,7 @@ class PluginReminderGateway extends ReminderGateway {
     final channel = channels
         ?.where((item) => item.id == 'hepan_lessons')
         .firstOrNull;
+    final appEnabled = await _android?.areNotificationsEnabled();
     Map<Object?, Object?>? system;
     String? systemError;
     try {
@@ -338,6 +361,11 @@ class PluginReminderGateway extends ReminderGateway {
     }
     return {
       'notifications': access.notifications,
+      'appNotificationsEnabled': appEnabled,
+      'channelExists': channels == null ? null : channel != null,
+      'courseChannelEnabled': channel == null
+          ? null
+          : channel.importance != Importance.none,
       'exactAlarm': access.exactAlarm,
       'pendingCount': pending.length,
       'channelImportance': channel?.importance.value,