fix(task): disable Confirm button when no date is selected in absolute reminder picker

Prevents emitting a reminder with reminder=null/relativeTo=null when
the user clicks Confirm without selecting a date first.

Refs #2208

frontend/src/components/tasks/partials/ReminderDetail.vue

@@ -58,6 +58,7 @@
 						v-if="activeForm !== null"
 						class="reminder__close-button"
 						:shadow="false"
+						:disabled="activeForm === 'absolute' && reminderDate === null"
 						@click="confirmAndClose(close)"
 					>
 						{{ $t('misc.confirm') }}