Off-by-one day errors in Datepicker

**Describe the bug**
The DatePickerInput component is sending full ISO 8601 datetime strings (e.g. 2025-11-26T00:00:00.000Z) to the backend. This could cause off-by-one day errors when the UTC conversion shifted the date.

Fix here : https://github.com/Chainlit/chainlit/pull/2835