fix: use Date format (not DateTime) in clazz datesSet callback

The datesSet callback was formatting dates as "YYYY-MM-DD HH:mm:ss"
but the by_hty_id backend endpoints now use string_to_date which
only accepts "YYYY-MM-DD" format. Caused "trailing input" errors.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/pages/clazz/index.vue

@@ -651,8 +651,8 @@ export default defineComponent({
           })
           search();
         } else {
-          state.start_date = formatDate(start, DateFormatter.DateTime);
+          state.start_date = formatDate(start, DateFormatter.Date);
-          state.end_date = formatDate(end, DateFormatter.DateTime);
+          state.end_date = formatDate(end, DateFormatter.Date);
           search();
         }
       },