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/useEnsurePlaybackServerOnMount.ts

@@ -0,0 +1,33 @@
+import { useEffect, useState } from 'react';
+import { useAuthStore } from '../store/authStore';
+import { usePlayerStore } from '../store/playerStore';
+import {
+  ensurePlaybackServerActive,
+  playbackServerDiffersFromActive,
+} from '../utils/playback/playbackServer';
+
+/**
+ * On Now Playing surfaces, switch the browsed server to {@link queueServerId}
+ * before Subsonic fetches run. Returns false while a switch is in flight.
+ */
+export function useEnsurePlaybackServerOnMount(): boolean {
+  const queueServerId = usePlayerStore(s => s.queueServerId);
+  const queueLength = usePlayerStore(s => s.queue.length);
+  const activeServerId = useAuthStore(s => s.activeServerId);
+  const [ready, setReady] = useState(() => !playbackServerDiffersFromActive());
+
+  useEffect(() => {
+    if (!playbackServerDiffersFromActive()) {
+      setReady(true);
+      return;
+    }
+    let cancelled = false;
+    setReady(false);
+    void ensurePlaybackServerActive().then(ok => {
+      if (!cancelled) setReady(ok);
+    });
+    return () => { cancelled = true; };
+  }, [queueServerId, queueLength, activeServerId]);
+
+  return ready;
+}