Refactor help menu navigation functions for simplicity and consistency

[coderabbitai]

Problem

The help menu navigation functions (helpMenuListUp and helpMenuListDown) in src/internal/handle_modal.go have several issues that need to be addressed:

1. Too complicated - The navigation logic is overly complex and hard to follow
2. Can be simplified - There's significant room for refactoring to make the code more maintainable
3. Similar to sidebar up down - The functions appear to duplicate patterns from sidebar navigation, suggesting inconsistency in approach
4. Not unit tested - The navigation functions lack proper test coverage

Proposed Solution

• Simplify the navigation logic by following patterns established in sidebar navigation
• Reduce complexity and improve readability
• Add comprehensive unit tests for the navigation functions
• Ensure consistency with other navigation implementations in the codebase

Context

This issue was identified during review of PR #1011 which adds search functionality to the help menu. The navigation functions were modified as part of that PR but the underlying complexity issues should be addressed separately.

Reference:

• PR: feat: Add search functionality to help menu (#1009) #1011
• Comment: feat: Add search functionality to help menu (#1009) #1011 (comment)