Improve encapsulation by reducing visibility of public fields

[coderabbitai]

Description

As identified in PR #41, there are several instances where field visibility could be reduced to improve encapsulation.

Specific instances:

1. menuRegistry in MenuManager should be changed from public to private, with accessor methods provided as needed.

Rationale

Exposing internal state through public fields can lead to:

• Inappropriate modification of data
• Inability to validate changes
• Harder maintenance as usage can't be easily tracked
• Reduced ability to change implementation details

Recommendation

For each public field:

1. Change visibility to private (or protected where inheritance is needed)
2. Provide getter/setter methods with appropriate validation
3. If needed, add specialized methods like removeMenu(String menuId) and menuExists(String menuId)

Impact

These changes will improve code quality, maintainability, and reduce potential for bugs caused by inappropriate external modifications.

Context

Originally identified in PR discussion: #41 (comment)