[fix]会员页，尝试修复倒计时的毫秒位，可能出现不满3位，导致页面抖动的问题

lib/utils/count_down_timer.dart

@@ -91,8 +91,9 @@ class CountdownTimer {
     String twoDigits(int n) => n.toString().padLeft(2, '0');
     String threeDigits(int n) => n.toString().padLeft(2, '0').substring(0, 2);
 
+    int milliseconds = (duration.inMicroseconds ~/ 1000) % 1000;
     return "${twoDigits(duration.inMinutes)}:"
         "${twoDigits(duration.inSeconds % 60)}:"
-        "${threeDigits(duration.inMilliseconds % 1000)}";
+        "${threeDigits(milliseconds)}";
   }
 }