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/locales/en.ts

@@ -897,6 +897,8 @@ export const enTranslation = {
     notWithCrossfade: 'Not available while Crossfade is active',
     gapless: 'Gapless Playback',
     gaplessDesc: 'Pre-buffer next track to eliminate gaps between songs',
+    preservePlayNextOrder: 'Preserve "Play Next" order',
+    preservePlayNextOrderDesc: 'Newly added Play Next items queue up behind earlier ones instead of jumping in front.',
     trackPreviewsTitle: 'Track Previews',
     trackPreviewsToggle: 'Enable track previews',
     trackPreviewsDesc: 'Show inline Play and Preview buttons in tracklists for a quick mid-song sample.',