Add dedicated modal callbacks separate from button configurations (Meteor Admin Sdk)

### Technical TODO

## Problem
Currently, modal callbacks are tightly coupled to button callbacks. This means repeated modal triggers require custom button configurations each time, making the API cumbersome and inflexible.

## Goal
Decouple modal lifecycle callbacks from button configurations to provide a cleaner, more flexible API.

## Acceptance criteria
- [ ] Modal callback API is designed (onOpen, onClose, onConfirm, etc.)
- [ ] Modal-level callback handlers are implemented
- [ ] Backwards compatibility with existing button callbacks is maintained
- [ ] Documentation and examples are updated