fix(a11y): make Settings and the Downloads tabs keyboard-reachable

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.

frontend/src/services/keyboard-shortcut-service.ts

@@ -411,11 +411,9 @@ async function dispatch(action: string): Promise<void> {
             );
             break;
 
-        case 'tracklist.delete':
-            document.dispatchEvent(
-                new CustomEvent('shortcut:tracklist-delete'),
-            );
-            break;
+        // No `tracklist.delete`: it dispatched an event nothing
+        // listened for, from a binding Settings advertised as
+        // configurable.  See backend/shortcuts/config.go.
 
         // Panel-specific: autotag review.  The view listens for these
         // while it is the view on screen, and for nothing while it is