The four header nav tabs each owned a PopupWindow for their dropdown.
A PopupWindow grabs the pointer while open, so hovering a sibling tab
delivered no hover event and the menus could not be hover-switched.
Add a HeaderMenuState global (open-index, anchor-x, items, home-item)
and the NavMenuEntry struct to state.slint. Tabs now write their
identity into the global on hover/click instead of showing a popup;
the menu panel itself will be rendered by a separate in-window overlay.
Move NavMenuItem into its own file so the overlay can reuse it.
cc
vicrodh
b6f5631f51