feat(notifications): add IOSNotificationPayloadBridge for iOS cold start payload

apps/lib/core/notifications/ios_notification_payload_bridge.dart

@@ -0,0 +1,27 @@
+import 'dart:convert';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../../features/calendar/reminders/models/reminder_payload.dart';
+
+class IOSNotificationPayloadBridge {
+  static const String _key = 'pending_notification_payload';
+  final SharedPreferences _prefs;
+
+  IOSNotificationPayloadBridge(this._prefs);
+
+  Future<ReminderPayload?> getPendingPayload() async {
+    final raw = _prefs.getString(_key);
+    if (raw == null || raw.isEmpty) {
+      return null;
+    }
+    try {
+      final json = Map<String, dynamic>.from(jsonDecode(raw) as Map);
+      return ReminderPayload.fromJson(json);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  Future<void> clearPendingPayload() async {
+    await _prefs.remove(_key);
+  }
+}

apps/test/core/notifications/ios_notification_payload_bridge_test.dart

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+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:social_app/core/notifications/ios_notification_payload_bridge.dart';
+import 'dart:convert';
+
+void main() {
+  group('IOSNotificationPayloadBridge', () {
+    test('启动时读取待处理的 notification payload', () async {
+      SharedPreferences.setMockInitialValues({
+        'pending_notification_payload': jsonEncode({
+          'eventId': 'evt_123',
+          'title': 'Test Event',
+          'startAt': '2026-03-20T10:00:00Z',
+          'mode': 'single',
+        }),
+      });
+
+      final prefs = await SharedPreferences.getInstance();
+      final bridge = IOSNotificationPayloadBridge(prefs);
+      final payload = await bridge.getPendingPayload();
+
+      expect(payload?.eventId, 'evt_123');
+      expect(payload?.title, 'Test Event');
+    });
+
+    test('处理完成后清理 UserDefaults', () async {
+      SharedPreferences.setMockInitialValues({
+        'pending_notification_payload': jsonEncode({
+          'eventId': 'evt_123',
+          'title': 'Test Event',
+          'startAt': '2026-03-20T10:00:00Z',
+          'mode': 'single',
+        }),
+      });
+
+      final prefs = await SharedPreferences.getInstance();
+      final bridge = IOSNotificationPayloadBridge(prefs);
+      await bridge.clearPendingPayload();
+
+      final remaining = prefs.getString('pending_notification_payload');
+      expect(remaining, isNull);
+    });
+  });
+}