fix: initialize reminder channel and clarify notification permission diagnostics

mobile/test/widgets/reminder_tools_test.dart

@@ -70,6 +70,42 @@ Future<void> _open(WidgetTester tester, _Notifications notifications) async {
 }
 
 void main() {
+  testWidgets(
+    'Android 7 does not ask the user to create an unsupported category',
+    (tester) async {
+      final notifications = _Notifications()
+        ..diagnosis = Completer<Map<String, Object?>>();
+      notifications.diagnosis!.complete({
+        'notifications': true,
+        'appNotificationsEnabled': true,
+        'channelExists': false,
+        'system': {'sdk': 24},
+      });
+      await _open(tester, notifications);
+      await tester.tap(find.text('检查提醒'));
+      await tester.pumpAndSettle();
+      expect(find.textContaining('类别尚未建立'), findsNothing);
+      expect(find.text('系统通知总开关：已允许'), findsOneWidget);
+    },
+  );
+  testWidgets(
+    'diagnosis separates the application switch from the reminder category',
+    (tester) async {
+      final notifications = _Notifications()
+        ..diagnosis = Completer<Map<String, Object?>>();
+      notifications.diagnosis!.complete({
+        'notifications': false,
+        'appNotificationsEnabled': false,
+        'channelExists': true,
+        'courseChannelEnabled': true,
+      });
+      await _open(tester, notifications);
+      await tester.tap(find.text('检查提醒'));
+      await tester.pumpAndSettle();
+      expect(find.text('系统通知总开关：未允许'), findsOneWidget);
+      expect(find.text('课前提醒类别：已允许'), findsOneWidget);
+    },
+  );
   testWidgets('test controls distinguish immediate and background reminders', (
     tester,
   ) async {