feat(queue): preserve Play Next order toggle (#464)

* feat(queue): add preservePlayNextOrder setting + playNext store action

- New Track.playNextAdded flag (analogous to autoAdded / radioAdded).
  Stale flags behind queueIndex are harmless — only forward streak scan.
- New playerStore action playNext(tracks): tags incoming tracks and
  delegates to enqueueAt for unified undo + server sync.
- New authStore boolean preservePlayNextOrder (default false). When on,
  playNext appends behind the existing Play-Next streak (Spotify-style)
  instead of inserting directly after the current track.

* refactor(context-menu): centralise Play Next; add Settings toggle + i18n

- Replace 3 inline splice/enqueueAt call sites in ContextMenu with the
  new playNext action. Side-benefit: the single-song path now goes
  through enqueueAt and gets undo + queue sync (previously missing).
- Settings → Audio → Playback: new toggle below Gapless.
- 8 locales: preservePlayNextOrder + preservePlayNextOrderDesc.

* docs(contributors): credit + changelog entry for #464

src/store/playerStore.ts

@@ -73,6 +73,10 @@ export interface Track {
   size?: number;
   autoAdded?: boolean;
   radioAdded?: boolean;
+  /** Inserted via "Play Next". Used by the preserve-order toggle to find the
+   *  end of the current Play-Next streak. Stale flags behind queueIndex are
+   *  harmless — the streak scan only looks forward from queueIndex+1. */
+  playNextAdded?: boolean;
 }
 
 export function songToTrack(song: SubsonicSong): Track {
@@ -260,6 +264,12 @@ interface PlayerState {
    setProgress: (t: number, duration: number) => void;
   enqueue: (tracks: Track[], _orbitConfirmed?: boolean) => void;
   enqueueAt: (tracks: Track[], insertIndex: number, _orbitConfirmed?: boolean) => void;
+  /** "Play Next" — inserts after the current track. When
+   *  `preservePlayNextOrder` is on, appends to the existing Play-Next streak
+   *  (Spotify-style); otherwise inserts directly after the current track and
+   *  pushes any earlier Play-Next items down (default). Falls back to
+   *  `playTrack` when nothing is currently playing. */
+  playNext: (tracks: Track[]) => void;
   enqueueRadio: (tracks: Track[], artistId?: string) => void;
   setRadioArtistId: (artistId: string) => void;
   /** For Lucky Mix: drop upcoming tail; keep the currently playing item only. */
@@ -3247,6 +3257,23 @@ export const usePlayerStore = create<PlayerState>()(
         });
       },
 
+      playNext: (tracks) => {
+        if (tracks.length === 0) return;
+        const state = get();
+        const tagged = tracks.map(t => ({ ...t, playNextAdded: true as const }));
+        if (!state.currentTrack) {
+          state.playTrack(tagged[0], tagged);
+          return;
+        }
+        const baseIdx = state.queueIndex + 1;
+        let insertIdx = baseIdx;
+        if (useAuthStore.getState().preservePlayNextOrder) {
+          const q = state.queue;
+          while (insertIdx < q.length && q[insertIdx].playNextAdded) insertIdx++;
+        }
+        get().enqueueAt(tagged, insertIdx);
+      },
+
       clearQueue: () => {
         invoke('audio_stop').catch(console.error);
         isAudioPaused = false;