Fix Mantine 8 migration error with Date strings

frontend/app/components/dates/DateRangePicker.tsx

@@ -66,13 +66,13 @@ export default function DateRangePicker(props: DateRangePickerProps) {
         <DateTimePicker
           label="Start Date"
           value={startDate}
-          onChange={setStartDate}
+          onChange={(value) => setStartDate(new Date(value))}
           clearable
         />
         <DateTimePicker
           label="End Date"
           value={endDate}
-          onChange={setEndDate}
+          onChange={(value) => setEndDate(new Date(value))}
           clearable
         />
       </Group>