In fullscreen mode, the NSTitlebarAccessoryViewController buttons are hidden
with the system titlebar. This adds SwiftUI-based fullscreen controls that
appear in the sidebar area (when visible) or inline in the custom titlebar
(when sidebar is hidden), reusing the existing TitlebarControlsView component.
- Track fullscreen state via window notifications and toggle controls visibility
- Hide original titlebar accessory (isHidden + alphaValue=0) in fullscreen
- Route notification popover anchoring through fullscreen controls view model
so both button clicks and keyboard shortcuts (Cmd+Shift+I) position correctly
- Add debug titlebar spacing slider for fine-tuning leading inset
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