Show error message when start date/time is after end date/time

[quinise]

Description:
When users enter a start date/time that occurs after the end date/time, the form correctly prevents submission, but no visible error message is shown. This leaves users without feedback on why they cannot submit.

Expected Behavior:

• Display a clear error message (e.g., "Start time must be before end time") whenever the start date/time is after the end date/time.
• Keep the submit button disabled until the issue is corrected.

Steps to Reproduce:

1. Open the scheduling form.
2. Enter a start date/time that is later than the end date/time.
3. Attempt to submit.
4. Notice that the form does not submit but provides no explanation.

Labels: UI/UX, validation, enhancement