fix(audio): separate gapless chain from preload gate and reduce black flash

- Gapless now always chains at 30s regardless of preloadMode ('off' no
  longer breaks gapless playback)
- Byte pre-download (audio_preload) skips when Hot Cache is active to
  avoid duplicate downloads
- When both Gapless and Preload are active, bytes are pre-fetched early
  and the chain reuses the cached data (separate bytePreloadingId guard)
- player-album-art-wrap gets background: var(--bg-card) so the brief
  opacity-0 loading state shows a themed colour instead of black

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

src/styles/layout.css

@@ -1088,6 +1088,7 @@
   flex-shrink: 0;
   border-radius: var(--radius-sm);
   overflow: hidden;
+  background: var(--bg-card);
 }
 .player-album-art-wrap::after {
   content: '';