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/hooks/useConnectionStatus.ts

@@ -49,7 +49,7 @@ export function useConnectionStatus() {
 
   useEffect(() => {
     check();
-    intervalRef.current = setInterval(check, 30_000);
+    intervalRef.current = setInterval(check, 120_000);
 
     const handleOnline = () => check();
     const handleOffline = () => setStatus('disconnected');