perf: drastically reduce background API requests to Navidrome (v1.34.5)

- NowPlaying polling: only active while dropdown is open + respects
  Page Visibility API — was firing every 10s unconditionally (~8.6k req/day)
- Connection check interval: 30s → 120s (4× reduction)
- Queue sync debounce: 1.5s → 5s (prevents burst on rapid skip)
- Rating prefetch: add 7-minute in-memory TTL cache for artist/album
  ratings — repeated random album loads no longer re-fetch known ratings

Fixes server log flooding reported by users behind reverse proxies (Traefik).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/store/playerStore.ts

@@ -264,7 +264,7 @@ function syncQueueToServer(queue: Track[], currentTrack: Track | null, currentTi
     savePlayQueue(ids, currentTrack?.id, pos).catch(err => {
       console.error('Failed to sync play queue to server', err);
     });
-  }, 1500);
+  }, 5000);
 }
 
 // ─── Audio event handlers (called from initAudioListeners) ───────────────────