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/locales/de/queue.ts

@@ -28,6 +28,7 @@ export const queue = {
   shareQueue: 'Warteschlangen-Link kopieren',
   shareQueueEmpty: 'Die Warteschlange ist leer — nichts zu teilen.',
   emptyQueue: 'Die Warteschlange ist leer.',
+  crossServerEnqueueBlocked: 'Titel von einem anderen Server können der aktuellen Warteschlange nicht hinzugefügt werden. Beende oder leere die Warteschlange zuerst.',
   trackSingular: 'Titel',
   trackPlural: 'Titel',
   showRemaining: 'Restzeit anzeigen',