feat(lucky-mix): instant mix from your preferences

Lucky Mix targets ~50 tracks in one run: pick seeds from your most-played artists/albums and 4+ rated songs, add similar tracks, then fill the mix with random library picks, skipping anything you rated 1–2.
On start it trims the old “upcoming” tail so the queue does not show stale next tracks, starts playback on the first viable seed, routes to Now Playing, and can emit structured steps to the backend debug log.

src/components/Sidebar.tsx

@@ -68,6 +68,9 @@ export default function Sidebar({
   const sidebarItems = useSidebarStore(s => s.items);
   const setSidebarItems = useSidebarStore(s => s.setItems);
   const randomNavMode = useAuthStore(s => s.randomNavMode);
+  const showLuckyMixMenu = useAuthStore(s => s.showLuckyMixMenu);
+  const audiomuseByServer = useAuthStore(s => s.audiomuseNavidromeByServer);
+  const luckyMixAvailable = randomNavMode === 'separate' && showLuckyMixMenu && Boolean(serverId && audiomuseByServer[serverId]);
   const [libraryDropdownOpen, setLibraryDropdownOpen] = useState(false);
   const [playlistsExpanded, setPlaylistsExpanded] = useState(false);
   const playlistsRaw = usePlaylistStore(s => s.playlists);
@@ -95,8 +98,13 @@ export default function Sidebar({
     [sidebarItems],
   );
   const visibleLibraryConfigs = useMemo(
-    () => libraryItemsForReorder.filter(c => c.visible),
-    [libraryItemsForReorder],
+    () =>
+      libraryItemsForReorder.filter(c => {
+        if (!c.visible) return false;
+        if (c.id === 'luckyMix' && !luckyMixAvailable) return false;
+        return true;
+      }),
+    [libraryItemsForReorder, luckyMixAvailable],
   );
   const visibleSystemConfigs = useMemo(
     () => systemItemsForReorder.filter(c => c.visible),