Add onBlur callback support for form fields

[v1lling]

Adds an onBlur callback property to form fields that fires when a field loses focus (after editing completes). This complements the existing onChange callback which fires on every keystroke.

Use Case:
For text-based inputs, onBlur is preferred over onChange to avoid running callbacks on every keystroke. Use onChange for checkboxes and select dropdowns where each interaction is intentional.