修改近一月计算逻辑：从当月1号开始计算，而不是往前推一个月

backend/src/main/java/com/designhub/service/AnalyticsService.java

@@ -435,7 +435,8 @@ public class AnalyticsService {
             case "week":
                 return now.minusWeeks(1);
             case "month":
-                return now.minusMonths(1);
+                // 近一月：从当月1号开始计算
+                return LocalDateTime.of(LocalDate.now().withDayOfMonth(1), LocalTime.MIN);
             case "quarter":
                 return now.minusMonths(3);
             case "year":