feat(calendar): add ReminderQueueManager for handling reminder queue

apps/lib/features/calendar/reminders/reminder_queue_manager.dart

@@ -0,0 +1,31 @@
+import 'models/reminder_payload.dart';
+
+class ReminderQueueManager {
+  ReminderPayload? _currentPayload;
+  final List<ReminderPayload> _pending = [];
+
+  void enqueueFromClick(ReminderPayload payload) {
+    _currentPayload = payload;
+  }
+
+  void enqueuePending(List<ReminderPayload> payloads) {
+    payloads.sort((a, b) => a.startAt.compareTo(b.startAt));
+    _pending.addAll(payloads);
+  }
+
+  ReminderPayload? get currentPayload => _currentPayload;
+
+  bool get isEmpty => _currentPayload == null && _pending.isEmpty;
+
+  void dequeueCurrent() {
+    _currentPayload = null;
+    if (_pending.isNotEmpty) {
+      _currentPayload = _pending.removeAt(0);
+    }
+  }
+
+  void clear() {
+    _currentPayload = null;
+    _pending.clear();
+  }
+}

apps/test/features/calendar/reminders/reminder_queue_manager_test.dart

@@ -0,0 +1,60 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:social_app/features/calendar/reminders/reminder_queue_manager.dart';
+import 'package:social_app/features/calendar/reminders/models/reminder_payload.dart';
+
+void main() {
+  group('ReminderQueueManager', () {
+    test('按点击顺序处理，第一条处理完后处理剩余的按时间排序', () {
+      final manager = ReminderQueueManager();
+
+      final event1 = ReminderPayload(
+        eventId: '1',
+        title: 'Event 1',
+        startAt: DateTime(2026, 3, 20, 10, 1),
+        timezone: 'UTC',
+        mode: ReminderPayloadMode.single,
+      );
+      final event2 = ReminderPayload(
+        eventId: '2',
+        title: 'Event 2',
+        startAt: DateTime(2026, 3, 20, 10, 2),
+        timezone: 'UTC',
+        mode: ReminderPayloadMode.single,
+      );
+      final event3 = ReminderPayload(
+        eventId: '3',
+        title: 'Event 3',
+        startAt: DateTime(2026, 3, 20, 10, 3),
+        timezone: 'UTC',
+        mode: ReminderPayloadMode.single,
+      );
+
+      manager.enqueueFromClick(event2);
+      manager.enqueuePending([event1, event3]);
+
+      expect(manager.currentPayload?.eventId, '2');
+      manager.dequeueCurrent();
+      expect(manager.currentPayload?.eventId, '1');
+      manager.dequeueCurrent();
+      expect(manager.currentPayload?.eventId, '3');
+      manager.dequeueCurrent();
+      expect(manager.isEmpty, true);
+    });
+
+    test('单条通知处理完直接清空', () {
+      final manager = ReminderQueueManager();
+      final event = ReminderPayload(
+        eventId: '1',
+        title: 'Event 1',
+        startAt: DateTime.now(),
+        timezone: 'UTC',
+        mode: ReminderPayloadMode.single,
+      );
+
+      manager.enqueueFromClick(event);
+      expect(manager.isEmpty, false);
+      manager.dequeueCurrent();
+      expect(manager.isEmpty, true);
+    });
+  });
+}