Add useOnFormValueChange hook with Storybook examples

- Created useOnFormValueChange hook for reactive form behaviors
- Hook watches specific fields and executes callbacks on value changes
- Added comprehensive Storybook story with 3 examples:
  - Cascading dropdowns (country -> state selection)
  - Auto-calculation (order totals with discount)
  - Conditional fields (show/hide based on selection)
- Includes full TypeScript types and JSDoc documentation
- Added interactive tests for all story examples

Author: codegen-sh[bot]