feat(sidebar): toggle to pin Now Playing to the top (#1000)

* feat(sidebar): add toggle to pin Now Playing to the top

The fixed Now Playing entry can now be pinned to the very top of the
sidebar (above the Library label) instead of sitting above the bottom
spacer. Adds a persisted `nowPlayingAtTop` setting (default off, so
existing layouts are unchanged) and a toggle in the sidebar customizer
next to the Mix-navigation split. The entry stays non-hideable and
keeps its playing indicator in both positions.

* i18n(settings): Now Playing-at-top toggle strings (9 locales)

* docs(changelog): note Now Playing top toggle (#1000)

src/components/Sidebar.tsx

@@ -65,6 +65,7 @@ export default function Sidebar({
   const sidebarItems = useSidebarStore(s => s.items);
   const setSidebarItems = useSidebarStore(s => s.setItems);
   const randomNavMode = useAuthStore(s => s.randomNavMode);
+  const nowPlayingAtTop = useAuthStore(s => s.nowPlayingAtTop);
   const luckyMixBase = useLuckyMixAvailable();
   // Sidebar surfaces Lucky Mix as its own entry only in "separate" nav mode —
   // in hub mode it lives inside the Build-a-Mix landing page instead.
@@ -227,6 +228,7 @@ export default function Sidebar({
           handleNavRowPointerDown={handleNavRowPointerDown}
           isPlaying={isPlaying}
           hasNowPlayingTrack={!!currentTrack}
+          nowPlayingAtTop={nowPlayingAtTop}
           hasOfflineContent={hasOfflineContent}
           activeJobsCount={activeJobs.length}
           cancelAllDownloads={cancelAllDownloads}