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/authStore.ts

@@ -130,6 +130,7 @@ interface AuthState {
   preloadMode: 'off' | 'balanced' | 'early' | 'custom';
   preloadCustomSeconds: number;
   infiniteQueueEnabled: boolean;
+  preservePlayNextOrder: boolean;
   showArtistImages: boolean;
   showTrayIcon: boolean;
   minimizeToTray: boolean;
@@ -307,6 +308,7 @@ interface AuthState {
   setPreloadMode: (v: 'off' | 'balanced' | 'early' | 'custom') => void;
   setPreloadCustomSeconds: (v: number) => void;
   setInfiniteQueueEnabled: (v: boolean) => void;
+  setPreservePlayNextOrder: (v: boolean) => void;
   setShowArtistImages: (v: boolean) => void;
   setShowTrayIcon: (v: boolean) => void;
   setMinimizeToTray: (v: boolean) => void;
@@ -444,6 +446,7 @@ export const useAuthStore = create<AuthState>()(
       preloadMode: 'balanced',
       preloadCustomSeconds: 30,
       infiniteQueueEnabled: false,
+      preservePlayNextOrder: false,
       showArtistImages: false,
       showTrayIcon: true,
       minimizeToTray: false,
@@ -607,6 +610,7 @@ export const useAuthStore = create<AuthState>()(
       setPreloadMode: (v: 'off' | 'balanced' | 'early' | 'custom') => set({ preloadMode: v }),
       setPreloadCustomSeconds: (v: number) => set({ preloadCustomSeconds: v }),
       setInfiniteQueueEnabled: (v) => set({ infiniteQueueEnabled: v }),
+      setPreservePlayNextOrder: (v) => set({ preservePlayNextOrder: v }),
       setShowArtistImages: (v) => set({ showArtistImages: v }),
       setShowTrayIcon: (v) => set({ showTrayIcon: v }),
       setMinimizeToTray: (v) => set({ minimizeToTray: v }),