refactor(ui): extract layout app bars into AppBarContent

[luannmoreira]

Description

This PR refactors the application top bar by extracting shared logic and
layout into a reusable AppBarContent component and consolidating all
user-related interactions into UserMenu.

Both the admin and main UI now use the same app bar foundation,
reducing duplication and improving consistency across the application.

What changed

• Replaced inline v-app-bar implementations with AppBarContent
• Introduced slot-based left and right sections for layout flexibility
• Moved user actions (profile, menu items, dark mode) into UserMenu
• Added support and menu toggle events as explicit app bar capabilities
• Updated unit tests and snapshots to reflect the new structure

Reasoning

• Eliminates duplicated app bar logic between admin and main UI
• Centralizes common behaviors (menu toggle, support action)
• Improves maintainability and consistency for future UI changes
• Makes app bar features easier to extend without touching layouts

##How to test

1. Run the admin and main UI applications
2. Verify the following behaviors:
   • Menu toggle works on mobile view
   • Support button opens the ShellHub help page
   • User menu actions behave as before
   • Dark mode toggle works correctly
3. Run unit tests and confirm snapshots pass