Investigate the "ประเภทการอบรม" dropdown validation.

Current issue:
The form displays:

"Invalid input: expected string, received undefined"

instead of a friendly Thai validation message.

Tasks:

1. Review the React Hook Form configuration.
2. Review the Zod schema.
3. Review the Select component.
4. Ensure the field is correctly registered.
5. Ensure the Select binds:
   - value
   - onValueChange
6. Ensure defaultValues contains a valid default or empty string.
7. Replace raw Zod error messages with Thai validation messages.

Training Type options should be:

- INTERNAL
  Label: อบรมภายใน (Internal Training)

- EXTERNAL
  Label: อบรมภายนอก (Public Training)

- ONLINE
  Label: e-Learning / Online Course

- OJT
  Label: On the Job Training (OJT)

Validation message:

กรุณาเลือกประเภทการอบรม

Verify:

- Create mode
- Edit mode
- Reset form
- Validation after submit

Return:

- Root cause
- Files changed
- Fixed code
- Regression checklist