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/authStore.ts

@@ -227,7 +227,7 @@ export const useAuthStore = create<AuthState>()(
       minimizeToTray: false,
       discordRichPresence: false,
       enableAppleMusicCoversDiscord: false,
-      useCustomTitlebar: true,
+      useCustomTitlebar: false,
       nowPlayingEnabled: false,
       lyricsServerFirst: true,
       showFullscreenLyrics: true,