a11y.1 is the audit's last Critical and reproduced exactly: seven
config-section headers, seven bare `<div @click>`s with no tabindex,
no role and no aria-expanded, and every section collapsed by default —
so every setting in the app was behind a control that could not be
tabbed to. a11y.2 is the same bug in Downloads' two `<div class=tab>`s.
Both now follow patterns the app already had: a real
`<button aria-expanded aria-controls>` (explore-artist-details has five),
and a role=tablist/tab/tabpanel with a roving tab stop and
Left/Right/Home/End. The section body renders unconditionally and is
toggled with `hidden`, because aria-controls has to name an element
that exists and the slot's light-DOM children exist either way.
H-22's reorder ships with them: Libraries is first and the only
expanded section, Search Index — configured once, if ever — is second
to last. The Playback/Audio section H-22 also asks for is deliberately
not here: there is no output-device, gapless, crossfade or replay-gain
setting in backend/config to expose, and a section of controls that do
nothing is worse than admitting it does not exist.
Settings also stops advertising `tracklist.delete`, which was bound to
Delete and configurable in the UI while nothing listened for the event
it dispatched.