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/components/config-page/config-page.ts

@@ -163,12 +163,6 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
             scope: 'panel:track-list',
             defaultKey: 'Enter',
         },
-        'tracklist.delete': {
-            label: 'Remove Selected',
-            category: 'Navigation',
-            scope: 'panel:track-list',
-            defaultKey: 'Delete',
-        },
         'autotag.apply': {
             label: 'Apply Match',
             category: 'Autotag',
@@ -1500,14 +1494,20 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
         return html`
             <h2>Settings</h2>
 
-            ${this.renderSearchSection()}
+            <!--
+              Ordered by how often a setting is *reached*, not by how
+              the sections were written (H-22).  Libraries was last and
+              below the fold while Search Index — which is configured
+              once, if ever — was first and the only expanded one.
+            -->
+            ${this.renderLibrarySection()}
             ${this.renderNowPlayingSection()}
             ${this.renderThemeSection()}
-            ${this.renderFavoritesSection()}
             ${this.renderTrackListSection()}
+            ${this.renderFavoritesSection()}
             ${this.renderShortcutsSection()}
+            ${this.renderSearchSection()}
             <download-clients></download-clients>
-            ${this.renderLibrarySection()}
         `;
     }
 
@@ -1520,7 +1520,6 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
             <config-section
                 heading="Search Index"
                 description="The explore search index is built from the MusicBrainz/ListenBrainz data dumps — popular artists, albums, and tracks with listen counts — for fast offline search."
-                .open=${true}
             >
                 <div class="index-status">
                     ${s
@@ -2035,6 +2034,7 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
                 heading="Libraries"
                 description="Manage your music library folders. Scanning and its
                     progress live in the Jobs panel."
+                .open=${true}
             >
                 <div class="scan-actions">
                     <button