feat(challenges): 新增挑战鼓励提醒后台任务与通知支持

- 在 backgroundTaskManager 中增加 executeChallengeReminderTask，每日检查已加入且未打卡的挑战并发送鼓励通知
- 扩展 ChallengeNotificationHelpers 提供 sendEncouragementNotification 方法
- 新增 NotificationTypes.CHALLENGE_ENCOURAGEMENT 及对应点击跳转处理
- challengesApi 补充 checkedInToday 字段用于判断今日是否已打卡
- 临时注释掉挑战列表与详情页头部的礼物/分享按钮，避免干扰主流程

utils/notificationHelpers.ts

@@ -1,5 +1,5 @@
 import * as Notifications from 'expo-notifications';
-import { NotificationData, notificationService } from '../services/notifications';
+import { NotificationData, NotificationTypes, notificationService } from '../services/notifications';
 import { getNotificationEnabled } from './userPreferences';
 
 /**
@@ -287,6 +287,33 @@ export class GoalNotificationHelpers {
   }
 }
 
+export class ChallengeNotificationHelpers {
+  static buildChallengesTabUrl(): string {
+    return '/(tabs)/challenges';
+  }
+
+  static async sendEncouragementNotification(params: {
+    userName: string;
+    challengeTitle: string;
+    challengeId: string;
+  }): Promise<string> {
+    const { userName, challengeTitle, challengeId } = params;
+    const notification: NotificationData = {
+      title: '挑战提醒',
+      body: `${userName}，今天还没有完成「${challengeTitle}」挑战，快来打卡吧！`,
+      data: {
+        type: NotificationTypes.CHALLENGE_ENCOURAGEMENT,
+        challengeId,
+        url: ChallengeNotificationHelpers.buildChallengesTabUrl(),
+      },
+      sound: true,
+      priority: 'high',
+    };
+
+    return notificationService.sendImmediateNotification(notification);
+  }
+}
+
 /**
  * 营养相关的通知辅助函数
  */