fix(gantt): use reactive date range in Flatpickr config to prevent reset on task update

Replace static initialDateRange snapshot with reactive filters.value
references inside the computed flatPickerConfig. This ensures the
Flatpickr defaultDate always reflects the current user-selected range
instead of the mount-time values.

Ref #2462

frontend/src/components/project/views/ProjectGantt.vue

@@ -146,13 +146,11 @@ const flatPickerDateRange = computed<Date[]>({
 	},
 })
 
-const initialDateRange = [filters.value.dateFrom, filters.value.dateTo]
-
 const {t} = useI18n({useScope: 'global'})
 const flatPickerConfig = computed(() => ({
 	altFormat: t('date.altFormatShort'),
 	altInput: true,
-	defaultDate: initialDateRange,
+	defaultDate: [filters.value.dateFrom, filters.value.dateTo],
 	enableTime: false,
 	mode: 'range',
 	locale: useFlatpickrLanguage().value,