fix(unified-search): Send ISO date strings

Send expected ISO date string values for `since` and `until`.

See : https://github.com/nextcloud/server/blob/3d6699d685215e53ec51b65f491a6aece28e85b1/lib/private/Search/Filter/DateTimeFilter.php#L23

Signed-off-by: nfebe <fenn25.fn@gmail.com>

Author: nfebe

core/src/components/UnifiedSearch/UnifiedSearchModal.vue

@@ -441,8 +441,8 @@ export default defineComponent({
 					switch (filter.type) {
 					case 'date':
 						if (provider.filters?.since && provider.filters?.until) {
-							params.since = this.dateFilter.startFrom
-							params.until = this.dateFilter.endAt
+							params.since = this.dateFilter.startFrom?.toISOString()
+							params.until = this.dateFilter.endAt?.toISOString()
 						}
 						break
 					case 'person':