Desktop flattens the ARCHIVE popover into inline appbar links above the medium
breakpoint; mobile keeps the indented sub-list under ARCHIVE. GENRES and /tracks
removed from nav only — routes (GenresView, TracksView) remain reachable by URL.
Retires the now-dead desktop hover-popover and its 8.J collapse-state machinery
(mobile drawer still dismisses on click).
HashSet<string> _collapsedDropdowns replaces single bool so each parent
tracks its own dismiss state independently. onfocusout added alongside
onmouseleave so keyboard users get the dropdown re-enabled without a
mouse pass after Enter-activating a child link.
Add dd-nav-item-collapsed CSS class toggled on child click to override the
:hover/:focus-within show rules. Cleared on mouseleave so hover-to-open
works normally on the next pass. Mirrors the existing CloseMobileMenu pattern.
daniel-c-harvey