fix(playback): cross-server browse, Lucky Mix, and Now Playing (#768)

* fix(playback): keep browsed server when queue plays elsewhere

Lucky Mix on a non-playback server now clears the old queue and pins
the active server before building. Now Playing metadata uses
apiForServer against the queue server instead of forcing
ensurePlaybackServerActive on every activeServerId change.

* docs: note PR #768 in CHANGELOG and settings credits

* fix(now-playing): gate AudioMuse similar artists on playback server

Match getArtistInfoForServer credentials: when browsing another server
while the queue plays elsewhere, similar-artist count follows the queue
server's AudioMuse flag, not the browsed server.

src/hooks/usePlaybackServerId.ts

@@ -0,0 +1,18 @@
+import { useMemo } from 'react';
+import { useAuthStore } from '../store/authStore';
+import { usePlayerStore } from '../store/playerStore';
+import { getPlaybackServerId } from '../utils/playback/playbackServer';
+
+/**
+ * Subsonic server that owns the current queue / stream (may differ from the browsed
+ * server). Use for Now Playing metadata without calling `ensurePlaybackServerActive`.
+ */
+export function usePlaybackServerId(): string {
+  const queueServerId = usePlayerStore(s => s.queueServerId);
+  const queueLength = usePlayerStore(s => s.queue.length);
+  const activeServerId = useAuthStore(s => s.activeServerId);
+  return useMemo(
+    () => getPlaybackServerId(),
+    [queueServerId, queueLength, activeServerId],
+  );
+}