fix(ui): enhance menu behavior and item visibility

[luannmoreira]

Description

This PR enhances the behavior and visibility management of menu items in the AppLayout component. The changes focus on dynamically enabling or disabling items, improving child filtering logic, and ensuring more accurate state handling.
Key Changes

Dynamic Menu Item States:
    Introduced a disableItem function to disable menu items based on specific conditions (e.g., namespaces or user permissions).
    Applied the :disabled attribute to relevant menu items.

Filtered Child Items:
    Added a getFilteredChildren function to exclude hidden child menu items from rendering.

Namespace Handling:
    Updated the namespacedInstance computed property to better control the visibility of the "Billing" submenu.
    Simplified and replaced hasNamespace with namespacedInstance logic.

Reactive Items:
    Converted items from a static array to a reactive object for better state management.

Snapshot Updates:
    Updated AppLayout.spec.ts.snap to reflect changes in the rendering of disabled menu items and filtered children.

Impact

Improved User Experience:
    Disabled items provide clearer feedback for unavailable actions.
    Hidden items no longer render unnecessarily, reducing UI clutter.
Simplified Logic:
    Consolidation of visibility and state-handling logic into reusable functions.

Testing

All related unit tests and snapshots have been updated to ensure compatibility with the changes.
Manual testing was conducted to verify the following scenarios:
    Items are correctly disabled based on namespace conditions.
    Hidden child items are properly excluded from the DOM.
    "Billing" submenu visibility reflects namespace state and environment variables.

How to Test

Navigate to the application and verify that items like "Billing" adhere to the updated visibility rules.
Check that menu items are disabled or enabled as expected based on user permissions or other conditions.
Ensure that hidden child items are no longer visible in the DOM or interactable.

Fixes

This PR fixes #4417