[new]调整为15分钟

lib/data/repositories/member_repository.dart

@@ -59,7 +59,7 @@ class MemberRepository {
       return;
     }
     _isActivityCountdownRunning = true;
-    activityDuration.value = Duration(seconds: 20);
+    activityDuration.value = Duration(minutes: 15);
     _timer = Timer.periodic(const Duration(seconds: 1), (_) {
       final time = activityDuration.value;
       if (time != null) {