Show hidden menu bar items in a separate bar

## Summary

Add an option to show hidden menu bar items in a separate bar below the macOS menu bar, instead of only expanding them inline inside the normal menu bar.

## Motivation

Hidden Bar currently expands and collapses items inside the system menu bar. That works well for reducing clutter, but it can still be constrained by limited horizontal menu bar space, especially on displays with a notch or many menu bar apps.

A separate bar below the menu bar gives hidden items their own row and makes them easier to access without relying on the system menu bar having enough horizontal space.

## Desired behavior

- Add an opt-in preference, for example `Show hidden items in separate bar`.
- When enabled, clicking the Hidden Bar expand icon shows hidden menu bar items in a separate bar below the menu bar on the current display.
- The separate bar should show a horizontal row at minimum.
- The existing inline expand/collapse behavior should remain the default when the preference is disabled.
- If permissions are required for item capture or click forwarding, the app and documentation should make that clear.

## Reference behavior

This is similar to the user-facing separate-bar behavior in apps such as Thaw/Ice. This request is only about the behavior; it is not asking to copy GPL implementation code.

## Acceptance criteria

- A user can enable a separate hidden-items bar from Preferences.
- Hidden menu bar items appear below the main menu bar when expanded.
- Existing inline behavior continues to work when the option is disabled.
- The behavior is documented, including any Screen Recording or Accessibility permission requirements.