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

app/challenges/[id].tsx

@@ -139,6 +139,7 @@ export default function ChallengeDetailScreen() {
   const progress = challenge?.progress;
 
   const rankingData = useMemo(() => challenge?.rankings ?? [], [challenge?.rankings]);
+
   const participantAvatars = useMemo(
     () => rankingData.filter((item) => item.avatar).map((item) => item.avatar as string).slice(0, 6),
     [rankingData],
@@ -296,11 +297,11 @@ export default function ChallengeDetailScreen() {
             tone="light"
             transparent
             withSafeTop={false}
-            right={
-              <TouchableOpacity style={styles.circularButton} activeOpacity={0.85} onPress={handleShare}>
-                <Ionicons name="share-social-outline" size={20} color="#ffffff" />
-              </TouchableOpacity>
-            }
+          // right={
+          //   <TouchableOpacity style={styles.circularButton} activeOpacity={0.85} onPress={handleShare}>
+          //     <Ionicons name="share-social-outline" size={20} color="#ffffff" />
+          //   </TouchableOpacity>
+          // }
           />
         </View>