fix(playback): pin queue playback to source server when browsing another library (#717)

* fix(playback): pin queue streams, cover art, and library links to queue server

When the active server changes while a queue from another server is playing,
keep streams and UI on queueServerId; switch back for artist/album links and
queue or player-bar context menus.

* fix(playback): switch to queue server when opening Now Playing

Ensure active server matches queueServerId before Subsonic fetches on the
Now Playing page, mobile player route, and queue info panel; scope caches
by server id.

* docs(credits): mention Now Playing in PR #717 contribution line

* fix(playback): route scrobble and queue sync to queue server

Address PR review: apiForServer for scrobble/now-playing/savePlayQueue,
clear queueServerId on server removal, mini-player queueServerId sync,
block cross-server enqueue with toast, and regression tests.

src/hooks/usePlaybackCoverArt.ts

@@ -0,0 +1,17 @@
+import { useMemo } from 'react';
+import { useAuthStore } from '../store/authStore';
+import { usePlayerStore } from '../store/playerStore';
+import { playbackCoverArtForId } from '../utils/playback/playbackServer';
+
+/** Cover art for the playing queue — uses {@link queueServerId} when it differs from the browsed server. */
+export function usePlaybackCoverArt(coverId: string | undefined, size: number) {
+  const queueServerId = usePlayerStore(s => s.queueServerId);
+  const queueLength = usePlayerStore(s => s.queue.length);
+  const activeServerId = useAuthStore(s => s.activeServerId);
+  const servers = useAuthStore(s => s.servers);
+
+  return useMemo(() => {
+    if (!coverId) return { src: '', cacheKey: '' };
+    return playbackCoverArtForId(coverId, size);
+  }, [coverId, size, queueServerId, queueLength, activeServerId, servers]);
+}