fix(page): make Cmd+L navbar auto-hide on mouseleave like hover
Previously, navbar opened via Cmd+L (source: 'shortcut') stayed visible
forever because scheduleHide() only ran for 'hover' source. Now both
'hover' and 'shortcut' sources auto-hide when the mouse leaves the
navbar area, with the same 300ms delay and re-enter cancellation.
The 'loading' source still only hides when loading completes.
Add test verifying Cmd+L navbar auto-hides after mouseleave.